I don't think it's the sunscreen. Try this test: paint your nails and let them dry completely. Spray your feet with OFF a couple of times. For me, it's the OFF. I finally figured it out this last trip.
They sell a non yellowing top coat almost everywhere. Mainly used for french manicures but works great on other colors as well. Clorene and sun can do awful things to finger nails and toe nails. Always good to carry your color and the top coat with you for long extended vacations.
I've been successful at having my pedicure (done by myself at home) last nicely for a two-week STJ vacation. Base coat; 2 coats of colour, 1 good quality top coat. I always let each coat dry thoroughly before adding on top of it.
I have tips and the last visit the polish got all gummy. When I told my girl, she said I need to tell her so she uses a different topcoat. Went to Cancun and told her..no problem. May want to check with a nail salon. I never have a problem with my toes..they stay on all week.