Is plane the best option if I get to Sanya in Hainan? I have no credit card, so how much does it cost if I bought a one-way ticket at the airport in Guilin? Is this wise?
There's one daily non-stop flight KWL- on Hainan Airlines. On Wed and Sun, it leaves 11:15am, on other days, it leaves 9:00pm. Flying takes an hour, much much faster than any other means.

One-way economy fare ranges from 350yuan to 780yuan for a full "Y" fare. Go to a travel agent in Guilin (not the airline counter at the airport) to get a discount fare. Should be closer to 350 than 780 on most time of the year.

Credit card makes no difference. The travel agents probably will prefer cash anyways.
No matter where you are , ship? The answer is NO, unless you have plent of time. The best option is fiying to Hainan, Fly to Sanya.

So much I had told you, hower, with yourself. And which will you choose.