Depends on the resort, some are/some aren't. Here are some examples:
GF Main Pool - Open, anyone could walk up (someone correct me if I'm wrong on this one)
GF Other Pool - Behind the pool gates
Poly Lava Pool - Barefoot Bar is partially open to public/part behind the pool gates
Poly Oasis Pool - Behind the pool gates
CR Pools - Pretty sure there is no pool barrier, anyone could walk up. (Although this bar gets two thumbs down in my book, not my favorite place)
YC/BC - Outside of pool area
Though, if I were going to resort hop, I wouldn't focus on the pool bars. I'd rather hit the lounges and restaurants bars. Better options for food, drink, seats, and overall merriment.
Trader Sam's, Tambu Lounge, Mizner's, etc.