My understanding is that the one-pack and two-pack paints are both polyurethane. The difference is the drying mechanism – one pack uses atmospheric moisture to catalyse, whereas the two-pack uses a reaction between the two parts. The one-part is theoretically less moisture-resistant as it tends to have small voids in it, caused by bubbles. Gel coat is polyester or epoxy.
I don’t intend to leave the boat in the water. But I was a little worried (unduly) that one-pack wasn’t waterproof enough even for a few days immersion.