There are large lockers and small lockers. The large lockers will hold a ton of gear (6 cylinders and several weight bags) but I'm not sure the door is large enough to get a typical roller bag inside. It sounds like you're planning to dive, go check in at 3, and then dive some more? Here are a few options.
1. One person watches the gear while the other goes and checks in.
2. Ask some responsible looking diver set up near you to keep an eye on your gear for the half hour it will take to go check in.
3. Set your bag near the air trailer and ask the staff to keep an eye on it. This may work better if you're renting a cylinder from them!
4. Lock your all of your heavy and/or wet gear in a large locker and roll the bag with the light stuff along with you.
Theft is always a possibility, but I leave a lot of gear on the ground at the park while I get food, fill tanks, use the restroom, or whatever. I haven't lost anything yet.