Sound like you need to take as much of the old paint off, like pressure wash/sand blasting, so the new paint will stick. Then put a base sealer on then repaint. I don't know if you put a oil base over a latex base. You stated moisture is a problem--have you talked to paint stores in the area/or at least a professional painter for suggestions. In Nor Calif, where they do have alot of ocean moister, they do use a good sealer first.

you really need to get the old paint off - thick coats of paint peel off - thin coats of paint stay on better - it has to do with compression and tension - paint is good in compression and not so good in tension - as it gets thicker it tensions and then cracks and then peels - a real good paint removal job is in order -

Using a latex after an oil base paint is a sure way to make it peel off. If there is latex already on the surface to paint, be sure to remove it before using an oil base.

if you have little children please have the surfaces tested for lead, simple procedure called xrf - then you can make sure to take proper precautions with peeling lead paint, and repainting

Try some Sherwin Williams Duration. If that doesn't hold up, hothing will!