Pat, as I recall from previous hull pressure...
washings done by the local yard it is around 1600 to 1800 PSI. That should take off most all marine critters without harming the previous bottom paint or the gelcoat. I'm assuming your boat has one or more coats of either hard or ablative bottom paint on the hull to help prevent marine growth.Once you finish pressure washing the hull below the waterline, scrape off any loose bottom paint, feather the edges where some old paint has come off, then lightly dry sand the bottom. I use the screen door type sandpaper from Home Depot on a swivel pad that attaches to the end of a painting pole that sheet rockers use to smooth out the wall board compound after it has dried.I use the one inch diameter dense foam paint rollers on the end of the pole to apply the paint, also from Home Depot. Two coats of ablative last me three seasons. I've been very happy with the several bottom jobs done this way. It's fast, very little mess, and except for the paint, low cost. Hope this helps.Terry