1: Shrimp compete with fish.
Here is an example of an arthropod body plan evolved to be a match with fish. Maybe not esoteric enough for your needs but the real deal.
2: Swimming bug x armored fish.
But let us get more awesome. We will start with the most water adapted insect I can think of: the Belostoma water bug.
And we will take a page from the playbook of the Devonian armorer fishes like Dunkles here.
Of course the armor is not bony plates but exoskeleton.
The result: a swimming bug with grabbing appendages, body armor and a wicked bite. These things would be too heavily armored to fly but the shrimp are ok with that.
Drawing to come, I hope!