When we went to Dunn's River Falls a few years ago, we booked our excursion with Peat Taylor and he was fabulous. He took us to the falls, on a short tour of the countryside and then back to the ship. In the afternoon, he even came back to take us shopping and that was included in the cost which I think was around $30 pp. In case you want to look him up, his website address is http://www.peattaylor.com
Lockers at the falls ....we did use the lockers. All of our stuff including my digital camera was stored in the locker with no problem. You can rent a lock for your locker for about $10 and I think you get $5 back when you return the lock.
About bringing a camera ... we used a couple of disposable waterproof/underwater cameras and it worked out really well since you will definitely get wet climbing the falls. Along the way, your guide will take pictures of you and they even have a spot where they take the official DRF pic--we bought ours, it's one of my favs from that cruise and I think it was only $7.
BTW, if you haven't bought water shoes for climbing the falls, I would strongly consider it.
One thing to beware of ... when you are leaving the falls, the vendors can be quite persistant so watch yourself.