Or, if we wanna minimise our damage to the seawater environment from the abovementioned chemicals we:

1. accept that the smell is natural biology.

2. make it the skippers first duty on arrival, to open the bog sea cocks and flush through a good few times. (and once more for luck)

3. when the boat starts it's rocky rolly thing we will expect more trapped H2S bubblings. So we give a final flush.

And all will be well thereafter.

the smell is  from the  sea water sat in the toilet ,,, if its sat for any amount of time the micro organism multiply and  it becomes very smelly  ,,,,

so,,, before leaving your boat  flush fresh water through the toiletsdo this a  few times to make sure the pipes get fresh water through them too,.,,

you should now find your toilets are not smelly any more

Hi Andy and welcome to the forum.

The sea water in the toilet and outlet pipe can easily be removed as you correctly say by flushing through with fresh water before you leave. When you return the heads will indeed smell OK.

The problem that is harder to resolve is the sea water / decomposing biomass that remains in the inlet pipe. (the biology has already been well described) The first flushing will suddenly produce a pungent pong as this contaminated water comes into the bowl. The toilet system will then need several flushings to get rid of this water and bring fresh sea water into the system.

That's one of boating's smaller hazards though and if anyone is too sensitive to withstand the nasty niff; they are almost certianly too sensitive to go to sea anyway.

Cheers

Chas