I call it the Stone Fish, because it looks quite a bit like one. Unintentionally, I was just building with the pieces I had and ended up with a short, squat, heavy looking submarine. I was actually trying to build some sort of star ship when I started but once I added the propeller engines, it looked a lot more aquatic.
That's the alien pilot from the Space Police set I used some pieces of this from in there behind the semi-truck like windshield. He has a control yoke you can sort of see and a gun clipped into a rack next to his seat, but as I didn't design a way to easily open and close the cockpit, he's in there for the long haul.
Side view. I imagine the long, swept back fin on the top is a communications antenna.
View of all the engines and turbines, thing must be pretty fast. Although, it's also heavy both in real life piece weight and in universe (whatever universe that might be) due to it's undoubtedly thick armor.
Here's your overhead. I didn't really put any weapons on it mainly because I couldn't figure out any halfway decent looking ones with the parts at hand.
And another shot of the engines and a bit of the hull. Note the little claws it rests on, those are used to grip surfaces so the sub can cling to unlikely spots and lay ambush to unsuspecting Atlantis guys. Whatever those divers are supposed to be doing in the Atlantis sets...
So there you go, give me a bucket of Legos and a couple days to fiddle with a design and this is the sort of thing I make. If and when I find the rest of my Legos, I'm probably going to pop the head off the alien pilot guy and replace it with the shark head piece of one of the shark man figures that are intended to menace the Atlantis divers but are too low tech to have built and subsequently piloted this thing. And so I can live out my Lego/Street Sharks crossover fantasies.