"Just making the sub shoot bullets doesn't make a boss", as the saying goes.

It would need to have a few hatches which some different forms of canons would pop out of and the center part would need to have a cracked surface that would fall apart and reveal the inner goodies of it which in turn should spew some evil/good looking balls of fire/laser.
A land-bridge would need to lower itself onto the ground and enemy troopers would pour out as cannon fodder.
And every now and then it would have to submerge and release a couple of missiles.
Yep, it sure would be fun to implement, but it's too time consuming for me at the moment.
Still no kid though, seem to be happy where it is. 'It' was due last sunday...