I did a lot of snorkling around Guam and that area of the Marianas Islands, Rota and Tinian. Lots of WWII artifacts can be seen snorkling but would need diving gear to actually do much exploring. As a side note, there is a lot of WWII artifacts on the ground too that you can get close to and as far as Rota is concerned, venture into the jungle and you may well trip over something not seen for 60 to 70 years. The water and land could still hold WWII munitions but they had a large scale removal from planes and ships under water in the 70s and 80s so the water wrecks are pretty safe from getting yourself blown up.
Most time spent in the water was around home on Guam. The North, West, and South had some amazing reef areas. The spiny starfish was getting to parts of them but it wasn't too bad. The bad was some of the currents. Reefs can shred you if you get pushed over them depending on the tides. Jagged corral is not your friend. The underwater wildlife is spectacular. Eels, clown fish, amebas, assorted tropical fish, big parrot fish, lion fish, stone fish, a big assortment of shelled creatures, clams, and more. Squid were a common sight morning and evening when scooling and were not hard to catch for a beach snack. Then there were the Nurse sharks who marked the limit you want to adventure over the reef. They never bothered anyone but beyond them, the reef went down and out of sight.
The East side of the Island didn't get much for Snorkling and very little diving. There was no reef and basically, you started at the top of the Marianas Trench so it just went down, down, down. Great for surfing though. There were a few very hidden caves where one could snorkle and from time to time come across nice lobster but if you were not a long time resident with local friends, you would never find the caves.
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