do I need a visa? Can I get vone in Sanya?

Paula, will your ship stop by any other Chinese cities? If so, you need to apply for a multiple L type in advance. If the ship just stops by Sanya and you stay less than 24 hours, you needn't apply according to the regulation because you can enjoy 24 hours free transit. However, the immigration might still require you to apply an entry permit in advance. So my suggestion is that you should apply a single entry one at least to play it safe.