Bella Thorne: BF's V-Day Treat ... Booty & Bikinis

Bella Thorne did her Valentine's Day duty for BF Gregg Sulkin -- and her gift was way better than some flowers or chocolate.

The couple celebrated at the Hard Rock Hotel Riviera Maya with daiquiris and, most importantly, some serious PDA in the ocean.

The 18-year-old Disney star looks pretty serious about the relationship -- booze (she's legal in Mexico) and a beach vacay will do that.