Check out the ROV section under boats on RCGroups.com.
Many people have built much more capable rovs for much less money
Large diameter ABS piping for the bodies, onboard power using a LiPo battery
usually, cat 5 ethernet cableback to a laptop
for control and imaging.
Complete control boards are available.
Brushless DC motors can run underwater with no protection.
The sailboat section is pretty cool as well, I have a couple small RC sailboats I keep onboard for playing at anchor
. Would love to try out a foiling RC cat!