You also have to remember, though, that units mostly represent more than one vessel AND also is an abstraction of both morale and damage resistance factors! This is why I would like to see hp rate from 1-20 or 1-50 rather than just 1-4 or 1-6! This way, you could lose up to 4 or 10 points of damage, respectively, and still heal even out in the ocean (as this represents morale and minor crew losses), between 10 and 25 points of damage, and you cannot heal just anywhere, but must AT LEAST be in the sea (as this would represent MAJOR crew losses and/or losses of vessels), once you get to between 12 and 30 points of damage, then only port and/or coastal repair is possible, as this is major damage and loss of vessels across the board! If your unit has anything less than 5 or 10 hp, respectively, then ONLY a port repair would be possible!
Yours,
Aussie_Lurker.