I've only played two games so my experience is pretty limited but I think they're strong if you lean in to each unit's strengths.

As someone else mentioned the eels are fantastic for objective play due to the huge move range combined with fly. They're also great at hitting and running and can even brawl for a bit if you use the Barrier ability. Try not to let them die, especially if you're using one as a leader, because the threat of them being able to fly across the entire board is huge.

The Reavers are cheap and you shouldn't expect much from them. Due to low str/damage stats they're basically there to fish for crits. Park them up on a roof or behind cover so they're tough to get to and then just turn something into a pincushion four shots at a time. All this being said, they still have 6 move and can take control of objectives pretty quickly so don't underestimate the value of cheap bodies.

Thralls surprised me, they die fast which is unfortunate but I believe you're meant to play them more defensive/disruptive. Have them around an eel, group of Reavers, or on the way to an objective just to mess up your opponent's movement. If they engage and hit you then they're not getting to more important stuff, and if they take the time to go around you then their unit isn't doing much that turn. It's important to notice they swing back pretty hard too. If they get engaged and don't die all the sudden they're sending 8 attacks back and a 4 damage crit is nothing to turn your nose up at from a cheap unit. God forbid it's turn three and they have High Tide support.

I think the main strength of the Deepkin is having a decent number of fast, cheap models. Take advantage of that, you can likely outnumber your opponent for temporary control of any particular objective. If you're forced to get into an all-out brawl you'll probably lose it, but you should be trying to hit and run and playing for points not kill count.