RE: Cheap disposable r/c boards
"Dynamic Diver" = a Submarine that uses adjustable fins (diveplanes) to propel itself under the water when enough forward momentum is obtained
or you could make it a static diver with an actuator pump or air tank (sucks water into a tank to dive, blows it out with air/pumps it out to surface)
a WTC (Water-Tight-Container) is often used to protect all electrical components, but bear in mind all subs have a critical depth and should you exceed that then it will be lost as the water pressure would crush it.
i tend to go for the mid-budget range components (like Towerpro Servos from ebay) so if you do happen to loose your sub your not out of pocket that much, but youve got decent gear to run with so theres less chance of any failures being equipment related