I have a 15 ft. alum. Boat.. I have to replace some of the plywood.. I was going to use pressure treated... can you paint PT. wood? Or should I use standard plywood?

Marine grade plywood would be best. Painting PT wood is tough because it has a high moisture content.

I can't speak about the moisture content issue, but unless you're concerned about termite damage, I can't think of any reason to use pressure-treated plywood/lumber in a boat. Pressure-treating refers to saturating the wood with chemicals that make the wood unappetizing to termites and doing so under high pressure to get the chemicals to soak in deeper than they would under normal atmospheric pressure. The sole reason for using pressure-treated lumber in construction is for this termite-resistance property, not rot-resistance. Since most termites come up from the ground, PT lumber is used for building areas contacting the ground (like soleplates), but not above that point.

Best to follow Amricca's advice and use marine-grade plywood. You'll be happier in the long run.