To be honest, it doesn't look all that much like a large creature thingy, it looks kinda like a scout or maybe the two-hit ship from the game Battleship. I forgot the name. Destroyer? I am just used to large things being long, tall, cumbersome looking. This looks fast, nimble, light to mid-range armaments and armor. I see the door on the side with a sort of observation porch, but somehow the scale still fails. Maybe it's so the ship's cat can get out and see the most recent conquest! =P

Although...

I'm imagining the thing longer, and instead of the top fin, it has the top half of zeppelin. Maybe an armored airship? Maybe it's capable of going in the air as well as the water? =3

Yeah... I'm not sure if it's clear, but I haven't drawn the whole thing. It's nominally whale-shaped, probably has quite a long "tail". I realised I had no idea how it was supposed to actually move and I'd already spent a while drawing details I didn't really understand so I kinda gave up haha

Well, whales are long things, but they also use much of their tails for propulsion. Since I doubt the crew would like to be bouncing up and down at full speed, or any speed, I'd say the tail would be a bit shorter, but to remain inspired by, or using the mechanics of wildlife, it would have to be segmented with large pistons / hydraulics moving the tail up and down. Perhaps with wheels, like a steam engine? Of course, you could also just keep it stationary, with a + style tail in the back with stationary engines. However, maybe the tail could be adjusted up, down, left, right, to assist in tight turns, along with the fin things up front.