What is this monster movie?

Cant remember it so brightly but Ill try to describe it as good as possible. I THINK movie is from early 80s or something. It happens underwater, there is some sort of a laboratory at the bottom of the sea. There is a room with a hole on the floor for the divers, and at the end this os where the monster sneaks into the room. If I remember correctly, in the end there were 3 humans who were fighting the monster and one of the humans was left behind with monster while the other two escaped. Laboratory then exploded. Monster was blue with big red (or black) eyes, it had two arms and two legs and big paddle in its back. This is all I can remember and I'd appreciate if you could help me find this movie I once saw in early 90s on VHS