Arrive Sanya on ship Jan. 31, 2019. Weather?

According to the former weather condition, the temperature may be around 20C. You need to take a thin coat.
As for places to go, you may go to Nanshan Tourism Area. You can also find many local dishes and snacks in the scenic area.
Enjoy your time!

Seafood is good in this place. Best restaurants in my opinion are near Mingzhu square on Hexi road (West road). Go to one of those which are opposite the Chunyuan Seafood square. All these places have been there ages. Yes simple restaurants, but taste is good & price value is good.

All fishes, crabs and mussels are alive in tanks. Pick what you like and check when they measure the weight. Be careful to understand how much the cost is (they can show you cost with calculator or use piece of paper as English skills are non existing of course). They often may use price per 500 grams. If you prefer your food not steamed then you need to have that instruction in Chinese (in piece of paper) to make them to get this point (e.g. fried grouper).

We will be there the same day, traveling on the Westerdam