They spotted the submarine and attempted to attack it, but to no avail. At least we think to no avail. What if a cannonball had been shot at it?
No cannons were fired, only small arms, although tests on a reproduction show that the tower could have been cracked by minie balls. But what’s interesting is that the evidence shows all the men died in their positions, and you’d think there would have been a scramble inside if it was filling with water. People simply don’t remain in their seats in that situation. More likely is that either the shockwave knocked them all out and the top hatch let in water or that they ran out of air and passed out, then sank to the bottom