[anonjohn]

Step back a bit for a moment.

-Why do you want her to skinny dip?
-Do you just enjoy innocent skinny dipping with friends in a non-sexual naturist/nudist manner?
-Is the swimming an "excuse" to see her naked? Why? simple curiosity? a frisson? or more?
-Do you want to have a sexual encounter with her?
-Do you think/hope that she wants one with you?
-If so, for how long? once? cousins with benefits? marriage?

She hasn't taken the baits you gave her. She clearly refused to join in at the clothing optional place and resisted a return visit. Were YOU naked there?

Was her skinny dip reference just a completely unserious random passing thought which she did not want you to progress? I know that my wishful thinking regarding someone I fancied, who was only being friendly or flirtatious, has led me to develop elaborate, imaginative and completely fantastical impossible scenarios based on hope and lust!

A lower risk way might be to go hiking (so SHE has a nice day) and then you get into the water without her seeing you naked eg "I'm going to skinny dip now, close your eyes if you must". But at all times she must feel free to do as she chooses and not feel threatened or pressured or manipulated in any way - otherwise she will probably recoil. If she joins in - great. If she does not, they you can both laugh it off, but you will have tried and will probably still have a cousin you can continue meeting at family events for the rest of your life.