How about a single person "boat" being an upgraded version of the Longboard, a freakin SURFBOARD omg I would love this and it goes perfectly with the longboard's skills.
2 Versions you could make, Cogwheel Surfboard and Windboard
Upgrade the Cogwheel Surfboard directly from the cogwheel longboard. 25-40 natural rubber (for a rubber tube that surrounds the board on water), 2 mechanical springs, 5 sturdy ingots (propeller), and Deeply Colored Oil for the "motor". Possibly a buoyancy controller or Wax nugget as well.
And then Trion could make some money here, add an upgrade ticket that lets you swap between Board mode and Surf mode so you can drive on both land and sea. This would obviously need to be made so the Surf can hardly move on land (longboards already get a drowning debuff)
For the Windboard it upgrades from the normal longboard and only needs a Non-stick polish ( or wax nugget), 5 fine lumber (mast) and 10 beautifully colored fabric (tiny wind sail). This version cannot be converted to land/sea and is just for sea.
It could be upgraded into the cogwheel surfboard though with the mats listed above plug a cog motor.