Proposed Canon
Ships are Water Capable vessels. They can carry Units across water hexes.
Ships have levels. They can reach at least level four.
Most Ships are useless without crews, the exceptions being self-powered examples created by Turnamancy.IPTSF Text 44
There may be different types of ships, or the type may change with levels. One type is the Galleon.
Ships may level up via experience earned in combat like other units, or they may have to be created at a particular level, or increase in some other way.
Ships may pop like other units, or may be created through a construction process, like Fabrication.
Ships may level by by being upgraded like cities.
Ships may Pop as Equipment along with a crew in the manner of Siege Towers.
 Carrying Capacity
A Lvl 3 Haggar ship carried twenty four Infantry Units, while two Level 2s and a Level 4 carried a total of seventy five Infantry, seven small Flyers, one Heavy Flyer, and a Warlord.LIAB Text 18
One possibility is limits of 10 units per level.