I second this. Make the prow and stern 'capstone' pieces that determined the ship width - either three tiles or five tiles. This concept worked well for round towers. In either case, the cargo hold hatches could be the middle tile, as could the mast pieces and lots of the midline rigging attachments.
This would be a hard kickstarter to fund compared to dungeons and caverns. It should probably be expanded out to include all of the needed ship accessories: wheel, coiled ropes, etc, as well as a docks system that integrates with city builder. I'd go further than that, including a shoreline system in a 4x4 footprint that tapers down to a battlemat underneath. If possible, we could do a line of sea caves that taper down to an ocean battlemat. I'd include a coral caverns set.
That's my aquatic adventures DF wishlist. I imagine pirate miniatures that are sculpted to be onboard ships... swinging from rigging, dagger gripped by the teeth.