I just tripped over this while searching for something else: Wayfair has an assortment of bidet seats, most of which aren't expensive. Although marketed for household toilets, they should fit any marine toilet that has a household size bowl. They all do need 115v ac power, but that's also true of those made for the marine and RV market. Worth checking out IMO.
https://www.wayfair.com/Washlet-C100-Toilet-Seat-Bidet-L807-K~TOT4855.html?refid=
--Peggie
https://www.wayfair.com/Washlet-C100-Toilet-Seat-Bidet-L807-K~TOT4855.html?refid=
--Peggie