Hi Leah,
That's why we're all here, no such thing as a dumb question. My kids have had their g-tube for two years (two summers at the beach). Their's is a bard button not the mickey, but they have never had a problem with sand. For asthetics, they like to keep it covered with a 2X3 bandaid at the beach. That might help for Reece if you're worried. If sand does get in it, just flush with water. You might want to bring a squirt bottle with you just in case.
As far as having it stay wet, its best to dry it off, or let it air dry. One of my sons has had no problem with his, the other has had granulation tissue around his site. We were told that it was from moisture. He was keeping it covered with a bandaid all day at school (he was afraid other kids would see it- jr high issue). His sweat would get caught in the bandaid so it would stay wet all day. The granulation tissue was really yucky, but we had it taken care of. The doctor cauterized it with silver nitrate. He now uses a powder (stomahesive) to keep it dry. No problems since.
Enjoy the summer- no worries.
Jane