Whoa this is such a cool idea, I absolutely love it!! Seconding Nez in that a downloadable version would have been very nice.
The whole mech idea is great, and the fact that it just moves on its own unless you stop makes you feel just a bit less in control, which works fantastically here. The scare is good, but it would be even better if there was more buildup to it. My main pains right now is that the lights are always on in the mech (making it feel quite safe really), and that there is very little visually showing you are actually in the world. I want to feel kind of fragile, like the water is actually just outside the mech. If it were to bob with each step, the camera shake a lil on each impact then it would feel so much more powerful and be more immersive.
I think you got a gem of an idea here, it would just have needed a lil bit different work. Not more. There is already more than enough stuff in the game, some stuff could be cut really (computer, radio, scanner) as they don't really make the game feel more terrifying. What has worked best for me is to pick one or two thing that I really focus on, and make as interesting as possible. Anyways, that's all my opinion, take it with a grain or seven of salt.
Overall, a really good idea, sound design was fantastic and mood was good. Great work!