Welcome to the Flow forum, @rjw! @Martydallas is trying to persuade you to go with the gold standard, hot wax dipping. He is absolutely right, that is probably the best possible hive treatment that you could choose. It is weather resistant, and will probably make your hives last for longer than you will. However, unless somebody in your local bee club has the equipment, it can be very difficult to organize.
You could use Tung Oil, but in warm and wet climates, it may not be the best choice, as it tends to develop mildew. I don’t think that weakens the wood, but it does discolor it it quite a lot.
Personally I would probably go for a high quality marine varnish. As long as you let it cure for a month or two before you put the bees into the hive, it will be non-toxic and long-lasting. Thin the first two coats 50:50 to allow it to penetrate properly. I would do at least 3 coats in total, and more if it looks patchy. Cedar is very thirsty wood, and it can be difficult to get an even appearance with fewer coats.
Hope that helps!