UV / Sunscreen

My partner and I are going away mid December, next week, to Cancun. I know it's typical for it to rain, but the UV is saying it's only going to be five? Is that normal? Should I still pack a super high SPF? We were hoping to come back with a tan and now I'm thinking I won't even need to wear sunscreen.

Update: overcast this entire week and haven't been tanned or burnt once. Decided to chance it today and yesterday with no sunscreen and with the UV at 5 I was not burnt! Or tanned! Maybe I'm lucky or the weather is just very, very bad.