Hey Cynthia! I suggest keeping the instrument in the case as much as possible and try to stay away from playing in any muggy heat. High humidity is not as bad as overly dry, so you really will only have a few potential issues. The violin will easily go out of tune as moisture in the peg box will build up and make the pegs slip. This doesn’t damage anything–just makes you have to tune more often. When you are tuning, make sure you push into the peg box as much as possible, as this helps them to stay for a longer period of time. Here is good lesson about tuning.
There is one extreme thing that can happen but I’ve only seen it a few times. Since there are two parts of the violin that are glued together, technically the violin can come apart if there is extreme humidity. Again I’ve only seen this a few times ever (once with the fingerboard, and once with the neck). The only time I’ve seen something like this happen is when a student left their instrument in their car in the 90 degree heat for over 18 hours.
A good rule of thumb would be to treat your violin as a human being. You wouldn’t stick a child in a car for 18 hours in the 90 degree heat I would hope? 🙂 Playing on the beach for a few hours won’t hurt it, you’ll just have to tune it a lot.