What if you had a compressed air tank you could switch on and seal off the engine compartment intakes? So say you had continuous air and didn't suck in any water when you went under, think it could happen then?
Just thinking out loud, but if you somehow did have an air tank under the hood, that you could activate, why would you need to close the intakes? Just have the tank release enough air to feed the engine, and excess to leave the compartment via the vents....
Just my thoughts, I could be overlooking something exceedingly simple.
I'm with you on getting them to sub....but I say if you can stab a straight suicide sub maybe 1000cc+ and sick pump would propel you up and out...if it didn't run out of air trying. It would have to rev hard and perfectly under the water for a couple of seconds. Mine accelerates bad under water because it seems to run out of air instantly.