1. formation of various molds and mildews on hidden undersurfaces, especially surfaces that cant be easily cleaned - underside of flooring, behind panels and ceiling, etc.
Get a mirror on a stick, plus a flashlight and search all the 'hidden' under-surfaces. If you find mold/mildew, CLEAN it, then apply 3M "Mildew block", or other 'strong caustic' detergent ... apply but dont wipe it off, etc. Be cleaning I mean 'removing it', not just killing it to become the nutrient source for other species of mold/mildew.

2. Where does your shower empty?? Skin cells from washing and showering that enter the open bilge, dissolve through 'putrefaction' and form odiferous chemical compounds known as 'putrenes' .... the same aroma that comes from 'rotting bodies'. If you're pumping 'wash water' into the bilge .... pump it direct overboard instead of into the bilge.
Putrenes permeate 'sanitary' hose faster than 'head stink'.
Nothing will make a boat interior smell as bad as 'rotting skin cells' in a bilge or bilge sump .... nothing!