Cabo has a semi-arid desert climate. Temperatures in December should be in the mid-70s to 80s. Since it is in the desert, temperatures might be a little cooler at night (mid-60s). Water temperatures should be in the mid-70s.
Most cell phones will not work in Mexico. Or when they do work, you will be charged $$$. You might want to turn data roaming off, too.
If you have an iPhone you may be able to iMessage others with iPhones. If you download Whatsapp, you should be able to text anyone for free (if connected to wifi) through the app while in Mexico.
Don’t forget to pack:
PASSPORT (might also want to make a paper copy)
Outfits for welcome party, rehearsal dinner and wedding
Your tux/bridesmaid dress/shoes if you’re in the wedding party
Your party pants