I've been here in Guam for two and a half years. Great dives in Guam - Blue Hole, The Crevice, Pete's Reef, Amphitheater, and many others. The key is to go outside the harbor. Once you get out there, the water is fantastic (visibility 100+). In the harbor are some wrecks, but visibility is hit and miss. Water is awesome. Shorts and t-shirt down to 130+ feet. Never needed a wet suit here.
MDA is the right place - cheaper than GTDS (Guam Tropical Dive Station), which is expensive and now seems to cater to Japanese tourists. And MDA does better dive tours (at least lately).
You'll love the diving here. It'll spoil you in no time. Hope this helps.
Jim