I live and dive in Saipan. The diving here is mostly shore diving, altho there are quite a lot of boat dives available. Grotto is recommended for turtles, esp at night and early a.m. (as they sleep in the Grotto), barracuda, occasional shark and lots of tropical fish. Vis is about 70 ft when the sun is out...great for pix. There are about 4 different dives available there. Great dive except the end, where you have to carry your tank up 100+ steps. Despite the steps...its highly recommended. Rated #2 dive spot by SkinDiver Magazine.
I personally like Obyan...also good vis. Eagle Ray City is great when the current is out as the rays love current. Bring your camera and a Palau dive hook. It is a boat dive usually combined with the Chen Seng Marau, a wreck dive which has lots of snappers and other fish. Both of these dives are shallow, abt 30 feet. there is a white tip that lives on the wreck.
Lau Lau, another shore dive has decent vis at times...altho the run-off frm the Lau Lau Bay golf course has messed up the vis. There are about 4 diff dives there. At the Pipe, vis can get up to 60-70 feet....nice plate coral there. In the Japanese cut, vis can be 15 feet on a bad day, but gets better once you get out further.
Wing Beach is a fav of mine...shore or boat, but only open a short period out of the year - Apr, May time. George, the large moray eel lives there, there are a couple of clown triggers, as well as a fire scallop. Other boat dives include Ice Cream and Dimple, as well as Naftan Point and Bonzai. Again, some sites are only open certain times of the year. The island of Tinian is a short boat ride away with good sites as well - Grotto, Fleming (wall dive) Dump Cove, leftover WWII stuff. The island of Rota requires a plane trip but vis is excellent there. Can see the wrecks from the top which are 100 feet down. Big thyphoon hit in Jan and the cement dock, as well as lots of Rota was hit bad...so not sure how the diving is there now, as I have not dove there since the thyphoon.
I dive in a shorty dive skin with a rash guard underneath during the "winter". Do get cold by the time I approach 40-50 minutes..but not much more is needed..esp if you come from Puget Sound or other cold water places.
Hope this helps.