This may be a LOT late and all, but I have found a solution without having to change the game. There IS a solution that allows you to beat this map without relying on the subs being able to submerge.
* SPOILER ALERT*
All you have to do is destroy 2 Power Plants and NO Tesla Coils. That will be the trigger (power down for the defenses) for reinforcements (Transport Ship with Volkov and Scientist) to arrive.