The Scenario (by SpellBound)
Your hands are tied together, tight with rope. they are held so that movement is restricted to only a few inches in any direction. this is achieved by them being tied to another rope which flows down form your hands and down underneath you, via your vagina. this then goes up, under your back and behind your head and is fastened securely, as i have just said, a few inches movement only.
 Your legs. these are also tied together at the feet with rope, they are underneath your body, so you are kinda crouching, and these are tied, as your arms were. so the rope from this goes to where your arm rope is securely tied.... your toes are also fastened together, big toes that is.
 You are blindfolded and ball gagged also.
 You are tied tight just under your breast and around your hips, with the ropes coming out sideways to restrict your lateral movement.
 You have an ice release key, that is, whatever you want to use to get out from this. it is suspended from the ceiling, or at least high, about six inches higher than the highest point of your body, and around two foot ahead of your knees.
And, this is possibly the bit you'll go, 'uh, no' to, you're in the bath, with the tap running.
Your mission, escape with the minimum amount of water. The water will, if warm, obviously aid the melting for your release, but by that time, much water will have passed under you.....
 It's probably either unfeasible or crap, or both.....
 Just, well, let me know what you think, i was quite pleased wih it, but to a seasoned pro such as yourself, probably tres easy....... or just mad!
 anyhoo... i hope you reply soon

I found the position with legs folded under my thighs too painful; there was no attaching point outside of the tub either... Thus I had to make a few modifications to your plan. But it doesn't change the main idea.

To hang the ice-cube system I used a rope tied between the shower hook and the curtain bar; it was making a hyperbolic sinus curve and in the lowest point I would tie the stocking with the key; as there was no lifeline connected to it, I hooked a small chain to make some weight. This way, the key would directly drop on my chest once the ice would have melted.

I made me nude and locked my cuffs around my wrists. Around my waist, I tied the middle of a three-meter rope with a loop just over my navel. In my back was dangling the remain of the rope in two long strands. I put my bite-gag between my teeth and tied it in place.

I then knelt into the bathtub and I used the remaining strands to tie my ankles against my thighs. My ankles were not tied together but they were quite useless anyway by now. I lied down in the tub, positioning my head on the lower end. My face would be slightly lower than my hips. The shower was laying in the tub too, at my feet. Just over me was suspended the key of my cuffs kept up by five ice cubes in a stocking; it was just over my chest and every moment I felt a drip of water falling between my breasts; strangely, it felt no as icy I had imagined...

With my head lying on the floor of the tub, I blindfolded me. This time I had decided to use an "official" blindfold, one I had received in a plane. It was a piece of red and smooth cloth, bean-shaped, held down by two elastic strands I had shortened to assure a better fit. I don't use it generally because I can too easily  get it off without my hands but here, on my back, in a slippery bathtub, it would stick on my face.

I pulled on the shower lever and opened the water. I had to put enough pressure to maintain the lever up and the water flowing through the shower. I tuned the water to be a nice temperature and I felt it showering on my legs and feet.

Everything was ready. I passed the connecting chain of my cuffs in the loop of the rope belt and clicked the master padlock. Now I had to wait the key to drop in order to escape the flowing water. It could seem dangerous as I could be drowned if the key didn't fall in time but in fact I knew I could ever keep my head above the water until I could flow over the border... It was not something I wanted to happen anyway...

In fact, this was really dangerous and I warn anyone who want to imitate me: if I passed out (as it could happen in a too hot water, I would probably be drowned... I have never passed out in a bathtub and I was thus pretty confident but who knows?

There I was, lying in the tub, feeling the warm water flooding it, its level increasing around my nude and tied body. It was very exciting. Tying my hands in front and preventing access to my pussy as well as to my breasts had proven to be quite impossible and thus I had full access to both of them. I could even touch my clit with a hand and one nipple with the other! How good it felt! I caressed myself but I was caring not to push me over the edge as I wanted to test the scenario until its end.

Water was going up rapidly, at least at the beginning in the bottom of the tub were the volume to fill is smaller. My feet were under the surface by now and I felt the water encircling my head. Blindfolded, the sensation was wonderful. As the water was just nice, it was difficult to tell where the surface was without touching it with my fingers. When my ears got immersed the silence was made all around me. It was quite strange and comfortable. The water was going up and up and up, slowly but irremediably. I kept perfectly immobile, letting my senses feel the delicious sensations: the water flooding above my neck, invading my waist... Only my legs, my curly pussy mound, my breasts and my figure were emerged by now. Soon, I felt the warm flood deliciously immersing my sex... My breasts seemed to be grasped by the smooth and warm hand of a lover; my nipples were all erect. I had opened my thighs and rested my knees on the wall of the tub; my right hand was gently rubbing my sex. It was as if I was floating in cotton dream, deaf, blind, warm, all turned on...

Now the water was attacking my face. I could feel it against my cheeks... Soon... yes! It was it! I felt a dribbling against my lips; the water began to flood my mouth... It was wonderful. I tried to shut of the opening but I couldn't close my lips around the large stick clenched between my teeth. I tried to spit the water but it was always replaced by more. I tried to swallow it but I couldn't gulp it down quickly enough to get my mouth free. And so I had to accept it: my mouth had to be flooded whatever I wanted; I was helpless to prevent it. I could feel water over my blindfold too by now. Only my legs and my nose were in open air.

Water came through my nose and I began to sneeze violently. I lifted up my head to be able to breathe normally again. How wonderful if felt to be here overflowed by warm water. I began to caress myself more widely. I wanted to arch back but then I could not touch my clit and my nipples simultaneously anymore and thus I had to keep my initial posture. I found that straining against the ropes that bound my legs was very exciting and I began to struggle fierily against my bonds. Blindly, I was immersing my head every few moments to let water fill my mouth as I loved the feeling of water entering it and then flowing out, helpless to prevent it in any manner.
I had no mean to estimate how much time there was before the key would be dropped down but I could touch that water line was near to the top of the tub. As I was floating in the water, there was no danger for me but it would soon make damages in my bathroom. I reached up with my chained hands and I could touch the key: as I was lifted up by the water, I was now able to get it without having to wait for it to be released!

I began to caress myself more fiercely, parting my nether lips, rubbing my clit and my nipples, fingering my pussy... Simultaneously, I was pulling on the rope, tensing my thigh muscles to the maximum. A few seconds of this treatment and I was coming like a female wolf, thrashing in the water, closing and opening my thighs frantically, groaning into my gag...

But I had no time to recuperate... Water was now dangerously close from the edge. Hoping that water had not damaged the mechanism of the padlock, I reached for the key and freed my hands. My first free move was to cut off the flow and then to remove my blindfold. How happy I was, there, sitting nude in my filled up bathtub, with my legs still tied up and the gag still parting my jaws!