it's gotta be possible if you manage to blobk every hole in ur case so the water wont get in, but then good luck if you need to change any computer parts. That goes as well fo your casing cooling, if there are no holes in your case you wont be able to put case fans and so ur temps might rise.
As for turning on the power, i have an option on my mobo where you can power on your comp by double cliking your mouse, but i dont think that can be done on every mobo.
And another thing, how are you going to manage with your cd/dvd drive?
and for your cable at the back, youd have to stick them up and you woudnt be able to take them out again.
anyway, that sounds like a cool idea, but if i were you id forget it just cause there are so many problems that would arise from such a project. what u could do though is buy a square doughnut tank and put ur case in the middle.
would that work for you?
Asus G1 Gaming Laptop