My boat has only a single
30 amp service, I run a 16K ac and a 5K ac as well as battery charger
, water heater TV's, multiple cell chargers, toaster, whatever. I can easily run the 16K AC and everything except the 5K AC, if I have the water heater on, 11 amp draw, I have the small AC off 5 amp draw.
With everything running including the water heater and 16K AC, but not the 5k ac I have a 26 amp draw, so far no problems.
On edit, the water heater only needs to be on for an hour or so a day it seems to keep water hot pretty well.
My 16K Ac is ancient and it's start up load is pretty big, but on shore power
I keep thinking of things, sorry. If you install a 220V heater element in a water heater running off of 110V you will decrease it's current
draw and of course it's heat output greatly, if I had a 220V heating
element I could run everything including both AC's, just barely, but as the heater needs to be on only for so short a period I haven't bothered