I am flying into SYX airport and staying at a nearby beach hotel. What would you recommend is the best way of getting to the hotel? Are cabs reliable there or do I need to arrange for some sort of car service?
I'm not sure if it's the best option, but on http://wikitravel.org/en/Sanya it mentions that the new train station in Sanya is far from downtown, approximately as far as the airport, and that a taxi to Dadong Bay (大东海), the most popular tourist area - you didn't mention what area you're staying in - there are 20km long beaches so a bit vague) is approximately ¥25-30.
So if that's roughly the same distance, that's probably about what you'd expect to pay by taxi, plus (potentially) an airport charge. A public bus for the same route is ¥1, so then that would be a more economical option from the airport two - Bus number 8 goes from the airport to Dadong Bay.
Another option is just to contact the hotel, they'll have been asked the question dozens of times and may have a simple option for you - maybe even a pickup if you're lucky ;)