I seem to recall the 12-15mm conundrum. Sadly can't recall the answer, sorry!
I also recall Screwfix sells a limited range of this stuff. Suggest buy a couple of fittings off them as a test?
Imho/experience...
Pumps. Worth adding a squirt of some sort of natural oil (i mean not mineral) and pump thru to help keep lubed, especially if leaving idle?
Commercially (especially with vacuum WC systems , like those on planes) uric acid build-up v problematic, can get as bad as concrete, needing harsh chemicals and a lot of work to shift. (I've seen it that bad, that have had to resort to "gun drills", to get initial acid circulation). Regular, Vinegar treatment probably enough for little boats?
Paper. We also found type of toilet paper to be critical in avoiding blockages. Some of this modern "luxury" stuff really wads up. You must use something that dissolves readily.
NO WIPES, of ANY kind! Any rig crew caught using wipes (we enlisted the cabin stewards as spies!) Were detailed to clear all blockages for the next week. Proved effective!
These people are experts.
https://www.leesan.com/homeHave a look at the info on pump lubes and descaling to get an idea.
Non functional bogs V rapidly become a BIG, expensive issue on a relatively small platform with 200 folk onboard..
Grandkids. Of course, based as we are in Cornwall the sun always shines here
....well maybe the sea is a bit warmer than the N sea, perhaps?
One of the best investments we made was kids wetsuits. The little devils play in the sea much longer then.
Yours must be spoiled by Singapore? I spent quite some time working in what was then a dockyard in Kepple (shopping centre now I believe!) And many happy weekends at Sentosa beach club, opposite.
I know where I'd rather swim!