May is pretty good. It is before typhoon season, so not likely to get blown out for a week when one of those things comes crashing through. Water temps are a bit colder than summer and fall, about 74-78 degrees F, 23-25.5 C in May. In my seven years there, January and February diving could be pretty abysmal because of weather, but you could generally find some place to dive. I still know some charter operators on the island and a civilian dive shop or two (or at least I did as of 2014 when I left). If you like wrecks, I would recommend the USS Emmons, a minesweeper/destroyer which sunk during the battle of Okinawa in 1945. BTW my avatar picture is at the Emmons site, and the hunk of stuff in front of me is the engine of a Kamikaze plane. Depth there is about 145'/44.2m. Lots of other shore and boat diving sites to be had. If we go it won't be until later in the year, and plans aren't solidified yet. Let me know if you are going in May, and I'll give you some names of a couple of charters and shops that I know of. It looks as though Simon knows of at least one as well.
The big guy in the attached video is Chuck Decesare, who runs North of Nago Dive Charters. I've been on his boat on many trips to the USS Emmons.