I would try a 100% CO 20% SF soap.
I made the same recipe and had a pair of sailors test it out for me while they were on a trip. They reported back that it lathered nicely and rinsed easily. They did also say the lather was on the creamier side, however they had a slightly undercured bar (4 weeks) and I had similarly creamy results at 4 weeks in the sink. It was bubbly in the sink at 10 weeks. Unfortunately, I don't have a comparison for natural salt water at 10 weeks as their bar is now sitting in their shower on land
I just pulled my tester out to try it with some salty water (35g salt mixed into 1000g tap water) and then tap water. I'd say the results are rather similar. Whether this works the same in proper fishy sea water is a different story that would require testing, but unfortunately I can't do that test at the moment due to poor local ocean water quality at this current point in time.
View attachment 70108
Salty water - Tap water
For transparency, this bar is 13 weeks old and is 100% CO 20% SF made with local sea water, coconut milk powder, and kaolin clay. Those ingredients may or may not contribute to or reduce the lather.
(Edit: Fixed the image!)