A must while in Jamaica! Both my husband and I agree that this is the best vacation activity we have done anywhere! When coming in off the cruise ship, you can take a taxi from the port ($15 round-trip per person) to go to the falls. Do not let the cab guy make you pay for the round-trip up front!
The falls were crowded, at least when we were there, but there was just a short wait to buy the entrance ticket ($12 per person). You may rent a locker for $8 and also water shoes, which we did not do as we had our own. There is a waiting area to go up the falls with a group, there was a guide there waiting for them but we went up on our own. The entrance is on the beach, and it is a strange sight to be standing at the ocean with a freshwater waterfall in front of you. This is the only bad part without a guide, you may want to join the end of a group just to get a spot to go up. Once past the first few climbs, the crowd thins out and being around a group is not necessary.
There are pools that are deep enough to immerse yourself in, the rocks have some hidden holes and can be slippery, but if you go slowly and watch your step, it is much more enjoyable to be alone. Try not to fall into the trap of the shops, we were told that they couldn't find their way out of them! Small kids are not a good idea as the water can move fast in parts; the youngest one we noticed was probably around 12. There is also a stairway along the falls where you will not get wet at all but can still walk along the falls, the people-watching here is entertaining!