User below helped me solve it, the movie is Shark Hunter (2001). Neat movie about a kid whose parents died when a science vessel sank under mysterious circumstances. After growing up and learning about marine biology and such, he gets recruited to explore the wreckage with a team that discovers the ship and his parents were attacked by a megalodon.

Disclaimer: I know it’s not Deep Blue Sea (1999). I also didn’t see the beginning of the movie, and only remember parts near the end.

So the movie is about a team of researchers on a massive submarine or something similar in the deep ocean. They get attacked by sharks that are larger than average, and aggressive to the vessel. There’s a moon pool big enough for a mini-sub to dock in.

A character supervising the moon pool walks onto a diving board and lays down on it to get a better view of something he sees. After laying down, one of the large sharks jumps up through the moon pool, taking the man and diving board in one bite before sinking back into the water.

Some details escape me, but the last 2 remaining characters hatch a plan, one escapes in a mini-sub while the other stays behind to kill the sharks by self-destroying the main sub. After the main sub explodes, killing the sharks, the person in the mini sub powers on their lights, only to discover an even bigger shark before arming their own self-destruct, before the movie ends.

At first I thought this might be one of The Meg movies, but I’m not sure those are correct. As I said, the moon pool scene is notably different than the one in Deep Blue Sea. I’m at a loss. I want to say it was on SciFi channel or maybe FX, but this was several years ago so I’m not sure.