There was a time when heads were simple.

A plank across the bows,  with circular holes.   Convenient in many ways.   Every time the ship dipped its head into the sea the fouling would be washed away,  both from the ship and,  presumably,  the men.   In those days sailors were almost all men.

These days the heads are much more complicated,  and they are hidden away in a tiny cubby hole below decks.

At home,  you press the button,  or the handle,  and your drinking water flushes away your waste.   On board,  you need to open seacocks,  move the lever,  operate the pump,  move the lever again,  pump again.

At home,  the system rarely blocks.   If it does,  you call a plumber and she sorts it out.   On board,   almost anything will block the heads;  the mantra has it "if you haven't eaten it,  don't put it down the heads".   The worst culprits are 'wet wipe' type things;  because they are made of cotton (or similar) they don't disintegrate in water or in the pump.   They wrap themselves around the valves,

 and they get trapped in the limescale in the pipes.   The pipes are narrower than those at home and they get blocked.

The 'limescale' is interesting (to a chemist!).   Human urine contains urate ions;  being associated with hydrogen,  sodium and potassium ions they stay dissolved.   Sea water contains calcium ions.   When urine mixes with sea water the calcium ions and urate ions combine to form calcium urate,  which is insoluble;  if left for any time in the waste pipes it precipitates on the walls of the waste pipes and gradually narrows them.   Over time the waste pipes become too narrow to pass anything that is not liquid.   Especially 'wet wipe' type things.   Less pleasant stuff,  too.

One answer,  of course is to flush properly.   The problem is partly that noise carries very well throughout a boat;  everyone can hear you pumping the heads,  especially at night.   So people tend to pump two or three times and then leave it.   It also leaves most of your waste still in the waste pipes,  where it accumulates,  settles and precipitates.   If everyone pumped 25 times (the recommended procedure) everything would end up in the sea and the pipes would be cleaned out (and everyone on the boat would be wide awake!).

When the heads become blocked you don't call a marine plumber;  do they even exist?   You leave it to the skipper.   When you own your own boat (or become a skipper of Nancy Blackett) you very quickly learn to strip down the marine heads and flush out the valves and pipes.

It's a nasty job,  but someone has to do it.