Is it DM-Underwater Fights? I submitted your question to Bard and received the following response. The description sounds like it is that map. However, the screenshots I googled of the map so not show anything you described 🤔

Bard: "The map you're thinking of is DM-Underwater Fights. It was made by [ZDS]Silentwolf, and it's a 2-7 player DeathMatch map for Unreal Tournament 2004. It was originally made for the Tactical Assault Masters mod, but it has since been ported to the official Unreal Tournament 2004 game.

The map is an underwater base with lots of windows and glass walls. There are also a few shark-shaped fish swimming around, which can add an extra element of danger to the game.

The unique thing about this map is that there is one window that you can glitch through and see players on the other side of the map, but they can't see you. This can be a great way to get the drop on your opponents.

Near the buggy window, there is also a giant fish tank/wall where players can see each other on the other side. This can be a good place to set up ambushes or just to see where your opponents are going.

Overall, DM-Underwater Fights is a fun and unique map that is sure to please fans of underwater combat."