I'm so jealous I could spit! I think that obviously...uumm water? Do not let your guitar get wet!!! I personaly don't take my instruments to the beach, the humidity, the sand, the naughty bloody little kids who want to touch it... But if you must, take very good care as the sun on your guitar is not great. And I would assume the humidity isn't great either. But I couldn't go without my guitar for a week... so I sympathise.
Have a cracking great time and take a dip for me, please!