In Hainan, with maps, everything is not as obvious as it would seem at first glance. For example, Google Maps does not work there, only through a VPN, you cannot immediately find Chinese applications on your phone, and even without knowledge of Chinese it will be difficult. Moreover, some maps are displayed in place with an offset (GPS shows the correct coordinates, but on the map you are in a different place), which is very confusing if you do not know about it in advance. I had a program on my phone Osmand with the downloaded map of China, it is not very detailed, but you can use.
I will briefly describe the situation with the cards. In fact, this is what happened to me. We walked around Sanya with Osmand, showed the location correctly. But at the same time, if I enter GPS coordinates in Osmand, taken for the point I need in a regular Google map, then the point is incorrect in fact. But if I find the right place on the Gogle satellite map and take the GPS coordinates for this point, then everything is fine, Osmand will show where it should be. That is, I conclude that the usual map from Google is shifted, you can use it not in GPS mode and you cannot take coordinates from it.
Ideally, if you go to China, you need to install the application on your phone in advance and order the necessary cards that work offline, so as not to mess with local Chinese applications and the cut-down Internet.