I constructed a shower in my boat a couple of years ago. I dont have any problems with mildew so far, or of smell. I dont have any problems with damp either - you just need to do a decent job and eliminate any problems like those as a part of the design process.
Hot water - where are you going to get enough water at the right temperature and sufficient pressure to make a shower work
this may need an increase in cold water tankage
I heat mine through a gas water heater
the standard hot water system provides enough pressure, and I vary the temp by actually varying the water temperature on the heater.
shower enclosure. My heads is big enough for a domestic shower curtain rail and for a standard nylon curtain to protect the rest of the heads (especially the loo paper). I sited this underneath a mushroom vent which I insist is open during showering to minimise condensation problems. The floor was a GRP moulding with a large wooden cut out for access to the stop cocks. I sealed the wood in having cut out a circular panel which I filled with a large dinghy access panel, and also fitted a domestic plastic shower drain with U bend. On top of the wood and surrounding deck, I installed a plastic grating material designed for swimming pool floors. On top of this (cause the grating was uncomfortable) I installed a caravan padded shower base material (v nice it is too).
Draining the water. I have already mentioned the standard shower drain and U bend, beneath this and in the bilge I installed a proper Johnson shower sump
and tapped into the wash basin outlet to pump out the grey water.
result a more than adequate shower at a minimal cost using a standard home shower spray (adjustable in height) and with as much space to soap and rinse as in a normal shower at home. After the shower I need to pass a couple of jugs of clean water through the drain to remove dirty residue (and stop subsequent smells) and also hang up the grating and shower base to dry underneath.
I can then move into the main dressing area, where I have an eberspacher (=espar) outlet to dress in comfort. (all this in a 29 ft boat (ok it is a cat))