Although not in the Pacific, how about Okinawa as an alternative? It's about half way between Japan and Taiwan, so it's more East China Sea than the pacific.
There are a number of islands and you can stay on the main island or several of the smaller islands.
In December, temperature will be around the low 20s (rather cooler than the UK is at the moment).
Or just to the south of Okinawa (about 300km - 185 miles - south) there is the island of Miyakojima. Lovely beaches there.
"What was there to do?"
If you're into snorkelling, diving etc then these are great places to go. Depending on the person, you may think that low 20s isn't warm enough to be on the beach but, for me, that's the ideal temperature.
On the main island there are historic sites to visit and there is an absolutely huge aquarium - one of the biggest in the world.
There are also lots of very good restaurants all over the place.