That's the whole point and fun of driving a craft on the water -NO Brakes.
I suggest, Emerg reverse, crash dive, hit A key (patchV1.2) as soon as you start diving and level off at 20 feet. -should provide enough drag. or...edit your subs .sim and zon files to provide more horsepower.
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All Ships can dive, but only Submarine's surface!"
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