Most boats, even bigger ones, use the heads as a wet locker. An Oyster
485 I almost bought had hooks inside the engine
compartment for that purpose. The boat I eventually bought does
have a real wet locker, with drains and a heat outlet inside it. In the climate where I sail (UK, English
Chanel, year round), we do get wet and the wet locker is a very desirable luxury. In warmer or dryer climates, it might be less essential.
I agree with others, however, about giving up space in the cabin
for water tanks. You have not only structure to think about -- but trim!! All that weight above the waterline is going to harm stability in a 28 foot boat. I would get that tankage lower, if I were you -- have you investigated flexible bladder tanks?