Since money is not an object, Eliminate the Macbook cases and build larger cases out of Titanium ( light weight and strength) and use the cases as large heat sinks since it will be submerged in water and the case can conduct the heat away from the interior utilizing more and larger fans.  Since the cases will be waterproof, I would vacuum all air and find a gas that will not condense as the computer is brought up or down into a cooler ambient temperatures.

Eliminate the hard drive for as much flash memory as you can buy and fit into the enclosure.  Shock proof the logic board and optical drives with Teflon or nylon springs.

Of course the weakest link in this whole scenario are the waterproof gaskets in the cases and your need to have waterproof 1/4 turn thumb screws. Not to mention sealing up around the screen even if you use a poly-carbonate covering.  As for the functional touch screen being "not a problem", I disagree.  To rugadize the screen you will need to put something in front of it, polycarbonate, lexan or even safety glass and you loose the touch screen capabilites.  Unless you use some kind of Wacom device.  I have yet to see anyone match up a electromagnectic screen with a Mac except for Wacom in their Cintiq line. Of couse I would love to see a Mac Mod with a electromgnetic screen.

I think you would have better luck in posting to the Navy Seals, they probably have a an Underwater Windoze Tablet that they spent 10 million dollars to develop and could probably sell as surplus for a hundred bucks.