I agree that ship combat is the easiest, fastest and most reliable way to level and make money with very little investment. Solo or not, PotD or not.
I don't even bother to buy another ship. I'll sink nearly all unique ships before char lvl 8 with a normal sloop - only upgrades are better cannons. For the Deck of Many Things I like to buy the Red Dream Hull upgrade (for safety reasons - don't want to reload ) and that's it.