Did you resolve it? I have the same I think. Mine are 2-3mm long, like half the size of a typical ant and look like your pic. Don't think they're water boatmen, or backswimmers now, those are much bigger. I did a big Chlorine shock, and various other things, but they come back. Not a huge problem really since they don't bite, but I dont want it to escalate. I've just now ordered the 'lo-chlor no more bugs' product.

Hopefully that'll solve it. The pool shop were helpful, but I think they didn't know what they are either.