I am thinking about using a pirate ship like stronghold and present it as an option to the players, but one thing struck me: at each stronghold level, strongholds get bigger, whether they be keeps or temples or establishments. A travelling barbarian camp can also expand to a larger size. However, you can't just "upgrade" your boat. Sure, you could get a bigger boat each time, but that means you're talking five different boats, and I don't want to make the players use a fleet. I'd rather find a way to feasibly upgrade their ship, without having to leave behind the old one (and its special qualities and sentimental value).

Any ideas?