It's only TOO cold for diving when the water takes on its solid form.
Where we dive around here, the water is normally 38F (3C) at the bottom. And it's enjoyable and fun - if you have the right gear.So, it won't be too cold for diving in Okinawa. It's just a question of what you think is too cold for you. What you want to do sounds pretty expensive. In that case, the extra cost to get a dry suit may not be that big a deal. And if you get the right, good quality dry suit, you can dive just about anywhere, any time, and be comfortable. There are divers here on SB that, from what I can tell, dive in a dry suit even in the very warmest water. Or maybe it's just any water where they wear anything more than just a bathing suit.Whatever you do, good luck and have fun!