I'm not even going to pretend to know how big this ship would be in minifig scale. Anyway, this is my Sea Offensive Power for Decisive Action.
About this creation
I'm not sure what class of ship this is, but it's most likely a battlecruiser. It sports 2 turrets with 2 18-inch guns, 6 40-mm Flak cannon, 8 torpedo tubes, and 2 depth charges. (At least 3 types of weapons visible, check) The torpedo tubes would be just above the waterline, with the depth charges below water.
Bridge, with a radar and communications dome.
Side shot close up of bridge and cooling system stack(it gets hot down below decks.) The blue stipe is for identification.
Bonnie has a magnetohydrodynamic engine.
Two rudders for more turning power.
It is shown in(technically on, but it travels IN water) plain blue water(check), and is built entirely out of Lego(check).