I was deployed to Guam back in 2003 through the summer and into the fall. It rains on schedule there in the morning as well as the afternoon. Cooler evenings and weak frontal systems will cause some intense rain but it doesn't last long.
Buy a Nikonos V with the 35mm lens for torrential shooting. These days they are almost free. Otherwise, just treat your camera as you would wherever you live. Make sure your camera bag has some sort of waterproofing. I took a Lowepro Stealth Reporter 500AW. it lasted through daily downpours, a tropical storm and one typhoon. Cameras always stayed dry when I intended them to.
When I was there, I had a Nikon F4 and a Nikon D100, both survived just fine, and I wasn't treating them with kid gloves. I was a working military photographer, out in all the weather at all hours, shooting the infrastructure projects that the US Navy Seabees were doing. On my days off, I was exploring.
If there is a tropical storm or typhoon, just don't go outside.
Don't slip and scrape yourself with coral. The wound will have to be debrided just like in a burn wound but the coral can grow inside your skin or blood vessels.
Buy as much fresh fruit as you can. You will get spoiled by coconut in Guam.
If you eat pork, find a fiesta and enjoy. Also, don't turn your nose up at grilled Spam with fresh pineapple.
Get a snorkel and some fins then go explore the lagoon. Take that Nikonos with. Don't set foot on the barrier reef or you could be washed out by a wave and never seen again like so many have.
Don't worry about the sharks, they probably won't eat you.
Hike to the top of the mountain.
Explore some caves. Take in the history there. If you have a chance, try to meet Debbie Peredo and ask her about her mother.
Enjoy Guam! It's one of those places that most people in the world will never dream of, let alone have the opportunity to visit.
Phil Forrest