In my mind, Palau would make the most sense. Delta has a direct flight from NRT to ROR. Then you could take United back home from there. Or go back to Tokyo. Plus if you are worried about how to get it expensed....then you could just pay for the Tokyo to ROR out of your own pocket, thus not screwing up your expense report.
As for critters...Palau isn't the best, It has some but not a lot. but it is still got great diving. Much better than Guam or Hawaii. You aren't going to great critters unless you go down to the Philippines or Indonesia.
Truk is also there too....best wreck diving in the world. But would be harder to get to than Palau.
Hope that helps.