Myles,
Here in Saipan most shops will only rent gear to you if you are diving with a guide or someone local. There are plenty of shops, but only about two if English is your choice of language. One is Korean, and the other is Russian, but English is pretty well understood at either.
There are three main shore dive spots, all good in their different ways. The Grotto is our #1 dive spot and should be considered an advanced dive. The two others are pretty easy shore dives and the area is large enough to do many dives and not get bored.
For boat diving, there are quite a few sites, though some are weather dependent. By May or June, conditions should be good for all the sites. From a boat, the best sites are on the North end or Southern end if the weather is permitting. The sites closer to the dock - central West side - can be quite good, but not so weather dependent.
PM me and I can give you more detailed or personal recommendations, and a couple web links with photos to check out.
Regards,
Dennis