During the part where Seagal is making an improvised bomb to damage the bow planes on the submarine, he is melting the explosive of a five inch starburst round by steaming. This is how it would be done - BUT!

A moment later, after his bomb had done it's damage, it is brought up that there are no high explosive five inch rounds aboard the ship. It was specifically stated that there were starbursts only. Nothing that would penetrate.

So, Seagal is melting the main charge out of a starburst round which has no explosive in it. It is an aerial parachute flare round used for illumination.

The only explosive in this round would be the detonator (nose) of the round which he is seen to remove and discard. So how could he make a bomb with the explosive force to disable a submarine when all he would have been able to remove from the projectile casing would have been the actual flare material and the parachute?

Think about it.