We have a saltwater pool and I swim a lot too.
I don't use anything special: I use normal shampoo and conditioner for the fake hair, which is usually products I'm just trying to use up. When I swim, I make sure to wear either a long braid or two pigtail braids, I never swim with my hair just "out". I wash the day I swim, but if I swim a lot in the week I hold off on the washing and just rinse my head really well in the shower until I do the whole wash routine, but I will put conditioner on the ends of the fake hair. I do the same with my leave out but use my staples that I use with my real hair. Because I have a saltwater I'm not too picky with the washing routine. If I swam in the ocean or chlorine I'd probably wash after each encounter.
I don't wear sew ins often, maybe once a year. Last summer was my family reunion and I wore a sew in and did swim a lot with the style, and this was all the things I did. hth