I love the fact that it’s compact and simple. I don’t know the setting and I don’t really have much experience, but here’s some stuff that quickly comes to my mind:

• Hidden tunnels that connect the rooms, maybe they allow the players to avoid traps/enemies or to enter the room in a stealthier way but maybe there’s a chance they get stuck, or only small creatures can fit inside.
• On one of the walls someone carved a picture: it shows a bunch of people killing each other and a weird creature watching in the background. The figures strongly resemble the party.
• On the bottom of the pond there’s actually another surface: when coming out of the water from the other side the characters find themselves in a teenage boy room, turns out the whole cave is the secret lair of a 15 year old wizard.
• One of the rooms is the inside of a giant skull. The creature is actually still alive and got stuck here thousands of years ago, it will offer something (knowledge? spells? ores?) to the players in exchange for ancient trading cards that could still be found in some monarch’s private collection, forgotten between crumbling ruins or selling for much less than they’re worth in a flee market.
• Goblins, but their community is being torn apart by adventurers coming here and killing them.

Yeah I like weird/gonzo stuff, how could you tell?

The problem here is you’ve got a small dungeon, so your pretty limited on what can reasonably go in it. Also, depending on your scale, the three secondary rooms are too close together. If there’s an encounter in any of them, it’s likely to draw every other encounter.

I’d suggest staggering the entrances, or maybe adding curves between them. Alternately, this could be a lair for 3-5 larger creatures, and each room is part of their life in some way. Maybe something amphibious that needs to breed in water.