We have a 50 liter calorifier
on our Moody 54 and it is enough to shower six people in comfort.
We have an Eberspaecher hydronic heating
system, as I imagine you also must sailing where you do (not the tropics, eh mate?) which is capable of putting probably 4 or 5 kilowatts of heat into the calorifier, besides heating
the boat. I switch that on in the morning (even if the boat doesn't need heating) before people start taking their showers. If I need a generator
run I do that at the same time and switch on the electrical
water heater element too. The electrical
water heater element is good because it is the only means of raising the calorifier up to its highest temperature. When the calorifier gets up to its max temperature, near boiling, it contains more heat energy and produces a greater volume of hot water (it's mixed with cold water as it comes out of the calorifier).
Having both the Eberspaecher and the electrical heating element on at the same time doesn't exactly heat the water as fast as people use it, but it comes close.
The other absolutely key thing to ensure abundant hot water:
We always instruct guests there are only 10 gallons of hot water on board. They can wash as long and as much as they want, but they must switch off the water between soapings if they don't want a cold shower at the end (sorry mates -- life at sea). If the guests do this, then water usage is cut by at least 2/3 and 50 liters is plenty for six or more people.
If none of this works I believe on your boat you could install a second calorifier to double your capacity.