First-time poster, first cruise was last week on the Voyager with a day stop in Falmouth. The town is definitely not "fully developed", and unless you're a seasoned traveler and comfortable finding your way around new (and "not fully developed") areas, you should probably pre-book your excursion to Montego Bay or the Falls. If you're only in port for the day, I don't think a trip to Negril is feasible, as it's about 2+? hours away.
We were told there's not much beyond shopping to do in Falmouth and that's fairly accurate if you're looking for snorkeling or beaches, etc. I spent an hour+ in Falmouth (outside of the port's secure shopping compound) sitting in front of a bakery with a few locals and had a lovely chat. The locals are friendly and thrilled about the new port, but when you first exit the port compound, you'll get hit with a stream of hawkers pushing tchotckes, hair braids and tour offers. It can be overwhelming, for sure.
If you do roll into Falmouth without booking a trip in advance, definitely look for a driver with Juta, and ask around for Donna (there aren't many female drivers). For $150, she drove us to a beach in Montego Bay, gave us interesting tidbits about the country, food, etc. along the way, and took me on a special side trip for a few foodie items I wanted to check out. $150 is steep, but if you split it with a group of 4-6, it's still $150.