Okinawa is known for its diving year round. Yes this is a tropical island, but the winter months (nov-apr) can get chilly. Most divers start to wear 3mm wetsuits around oct month and then shift into the 5mm wetsuits around dec/jan. For the true fair weather divers they jump into the Drysuit's.
Don't let the cooler temperatures stop you from diving. Always account for the water temps and even the surface winds. Most of the time you will be happy to get back in the water, but not thrilled to be standing on land during that surface interval.
Couple of suggestions to help you get through the winter months here in Okinawa:
1. always wear more layers for the dive and dry off once done diving.
2. Fill a cooler with warm bath water to pour down your wetsuit during the surface interval or upon completion of diving for the day.
3. Do not put on wetsuit/drysuit fully until ready to enter the water - still have to worry about over heating.
4. If your cold ---- End the dive and seek warmth!!!!
5. Always dive within your limitations
6. Plan for more use of air while diving.
Diving quite enjoyable here in Okinawa, just make sure you make it more comfortable for yourself while scuba diving here.