Aquatic underdark monster suggestions

Quick rundown on what’s going on, the party (a barbarian, Druid, rogue and ranger, all 5th level) has traveled into the underdark to investigate a cult stronghold. To get there they’ve had to get through a labyrinth of tunnels with all sorts of fun critters roaming through them. They’re now at the next leg of their journey, a massive underground petrified forest with a very wide river running through it.

To get where they need to go they’re going to have to go down the river in a boat, which I’m fairly excited for as there’s a lot of opportunity to build up tension and unease, as they can’t see what’s just under the surface. The stone forest was once part of a duergar kingdom, but they dug too deeply and the river burst forth, carrying much of the city with it, so they can expect lots of ruins as they travel further. I just can’t settle on what I want to throw at them. Not every encounter needs to be a fight, I’d just like some creatures that help build up the feeling of how alien the underdark can be at times, though smacking them around a bit is cool too.

I’ve played around with having an aboleth show up, but I can’t think of any real reason they’d be hanging around some old duergar ruins, they’re too smart for that.. unless they’ve gone insane I suppose. Any suggestions for aquatic underdark encounters are appreciated!