Does anybody know if there are any tours available in Sanya or where to arrange for a tour?

Sanya is a beach resort, not sure what you mean about available tours and arrangements.

We will be here for 1 day, arriving by cruiseship, and were wondering if it is easy to get a taxi to drive us to one of the parks or beaches or if there is any sightseeing. Thanks!

Here's a link to our trip report. On a reply I posted a list of some sightseeing on the island.
http://fodors.com/forums/threadselec...p;tid=34835590

Yes, you can get on a taxi and get to some sights like "The End of the World" or the Nanwan Island (Monkey Island) -- you will need to get with the taxi to a port where you can get on a cable car that brings you over to the island (a nature reserve).
You can check out my blog and entry about Sanya at http://site.chinafinds.com/blog/?s=Sanya