I think the answer is here....
" There has never been water found in the bilge until a boisterous sail last night, tacking into a 20kt wind. Found at least 10-12 liters there. It's only barely salty to taste."
I had exactly this scenario on the way to Tahiti some years ago.
Water sloshing around in the space under the aft head.
I tried and almost convinced myself that it wasn't salty, but it was......
Where did it come from? The seals on the rudder shaft.
We were on starboard tack in big seas for quite a while and the water pressure from the pounding forced water up past those compressed rings. It then worked its way along the port side under the aft bunk and went to the lowest point under the aft head.
Tightened up the nut and that stopped it temporarily.
Permanent fix was to replace the shaft sealing.
All the best