Lee and Noble board posters,

I make players use the build action to "muster" their ships. I never had
anyone trying to build more than two at a time so the issue of province level
never really struck me. Middle Ages navies were horrendously expensive so I
try to lean that direction.

You're correctg Ship maintenance is for the whole fleet at once. 1 GB per 3 units of troop the tubs can carry.