I say don't sell your jet ski. I primarily go swimming at the beach, but have access to the beach since I live in FL. I also go swimming at my dad's pool when we visit. I have been in lakes jet skiing and tubing. I don't do much to put my head under water and am just cautious. I haven't had any infections because of going tubing or jet skiing. I asked my docs about swimming and water activities post transplant and was told that to do what I felt comfortable with and that the transplant teams concern was my ingesting or aspirating water because of bacteria.
I would imagine that if you are wearing a life jacket when on the jet ski you would be fine, it would give you something to keep you afloat and your face out of the water if you were jet skiing or tubing and got thrown off into the water. Ultimately it boils down to what you feel comfortable with and what you are willing to accept as "acceptable risk." It took me about two years post transplant before I was comfortable swimming in a pool again because I was so nervous bacteria. The ocean never scared me though. Now I still don't swim much, so that may be a factor too how often are you going to be at the lake? Everything in moderation I say.
I personally wouldn't risk it straight away (like first 6 months) because you are still pretty immune suppressed and at more risk, but that would be a question for your docs.
Good luck with everything
Lindsey