Tee, here are a few things to consider:
How much mildew and condensation will you generate when showering? You don''t tell us where you are: big difference between Seattle and Miami, for example. You can deal with this if it''s a problem (e.g. use a dehumidifier) but it''s something to think about.
Where does the grey water drain? Apparently, into the bilge. Ugh... That''s going to be a big source of stink and growth. If you have a shower pan in one of the heads (you seem to suggest that), but it doesn''t lead to a separate shower sump (with its own pump), you should buy one. You can purchase a complete, off the shelf & self-contained unit from e.g. PAR/Jabsco and plumb it in for not much money. You need to send the stuff overboard.
Boats with two heads (again, that sounds like yours) will often set up one head for showering (full shower curtain, all the way around) and fall back on the other head for the balance of bodily functions. This won''t eliminate any mold/mildew issues for the head with the shower; it will minimize them.
Can you get water aboard all winter long? If in cold climates, dock water lines are sometimes drained to protect them. Not good to run out of shower water, not be able to make coffee AND no way to fill the tanks.
You do have a reliabile source of hot water, right?