@phxazcraig
Re: Saipan, Rota, and Tinian - I've dived all 3 of these northern Marianas islands. In general, if your going to be on any of these islands, it's worth a couple days of diving, but as a destination there are likely better locations relatively close by.
Saipan - "the Pipe" or Eagle Ray City is a dive worth doing if the conditions are right. On a good day (w/current) there can be 20-40 eagle rays that hang out in the current for your entire dive. Most divers just hold onto a rock to watch this spectacle, however I've heard a couple of folks use reef hooks as an effective anchor here (not sure what the local dive shops are recommending these days). This is still the best place I've dived with 10+ eagle rays consistently.
The other (somewhat) famous dives are caverns and a few wrecks. These are decent dives (vis. and temperature), but not really destination dives. There is a historically interesting site (Banzai Cliff) where hundreds of Japanese civilians committed suicide by jumping into the ocean instead of surrendering to American troops near the end of WWII. The weather has to be near perfect to dive this site - it is culturally significant to both the islanders and visiting Japanese tourists.
Rota - famous for the absurd underwater visibility. Seriously, it can feel endless (~250+ ft) in some locations (there is virtually no particulate in the water here, which has a downside as well - hard coral reefs are healthy, but not necessarily teeming with life). Best dive sites again are caverns and few wrecks.
Tinian - really a combination of both Rota & Saipan. There's an interesting wall dive, a few caverns, and some interesting reefs.
If you want to visit all 3 islands Saipan is worth 2 days of diving, and Rota and Tinian are worth about 1 day each. The flights between the islands are very short (<30 min) but flight schedules might interfere with your diving plans.
When I visited Saipan, we flew from Narita (the island sits between Japan and Guam). The plane was absolutely full of Japanese newlyweds (like the entire plane, many still in their wedding outfits). In addition to Japanese tourists, there are now more direct flights from S. Korea (same thing, honeymoon destination).
If you are interested in the region, I might suggest considering Pohnpei over the Northern Marianas. Pohnpei is culturally interesting (the lost ancient city of Nan Madol is here) and the diving is both more wild and more interesting. Palikir Pass (on an incoming tide) has hundreds of grey reef sharks (they used to dive this where we would literally drift through the wall of sharks to get into the pass). Several of the passes also host dozens of manta rays on incoming tides. The trip to Ant Atoll (off the coast) is worth the trip - untouched reefs, sharks, and someone recently reported they found a pygmy seahorse in one of the sea fans.
Palau - I've been several times, and will be returning next year w/my son. The Palau Pacific is a nice resort, and Sam's is a great dive operator. The only issue is that the travel time between Sam's and some of the best sites is lengthy (and in a few cases, prohibitive). You will spend 2+ hours/day "commuting" to/from the really good dive sites if you go with Sam's. It's a full day thing - you can do 2-3 dives (getting back late afternoon), or... you choose a liveaboard (like the Siren, the Ocean Hunter, or one of theAggressors) where you will get in 4 dives/day, have very short trips to/from the best dive sites, and generally have more time to relax between dives. They also get in the water at the best sites (like Blue Corner, Big Dropoff, German Channel, Peleliu Wall) before any of the day boats even get out to these sites.
I've done Palau both ways, and would recommend the liveaboard route to anyone interested in diving the best sites at the best times (and I'm a huge fan of Sam's, including how they operate, their kayaking tours, etc. The distance between their shop and many of the best sites is just lengthy).
