The ever elusive unicorn of the plant world.

What growing zone are you in?

Los Angeles.

Oh. In that case start googling Hollywood-style hedges. also, go visit a big giant nursery like Moon Valley and ask for expert advice.

Italian cypress look nice, but different from a hedge. I don’t think they would make a mess in the pool.

Search "plants for near swimming pools" and similar phrases and you will have a lot come up. Most recommended are those with jumbo leaves like bananas and palms that can be used only in mild winter climates. But this limitation will not be a factor where you are located.

It's impossible to recognize what's in the photo because it's so dark. I tried lightening it up, anyone have a guess?

Smooth pale bark may indicate a Ficus

Could be. Don't put a ficus if you don't like spreading, destructive roots, though.

Yes, it looked like a ficus to me as well… but the roots are awful for a pool. On a fence nearby we have a row of texanum but those have white flowers with bees in warm months so figured it’d be bad for lining the side of a pool.