I was on a treasure hunt once that placed poker chips among the rocky ledges of the dive site. Everyone that found a chip received a prize. I found a chip and promply turned it in. The only problem was that it was an undiscoverd chip from a previous hunt and didn't count. It was understandable but it sure let the air out of my excitement. I suggest you be prepared for the possible unexpected or unforseeable events by having extra prizes. Make sure the rules are spelled out clearly and everyone hears the instructions, such as when to begin, when to end, exactly what they are looking for, where to look, and who is allowed to recover which treasure. Have some items for the beginner that is off limits for the more experienced divers. Try to find ways to make it fun for all levels of divers. It's not uncommon for there to be a conflict or a complaint of unfairness. Just be ready for what you might do if the unexpected happens. A fun outing can lose its charm when a problem arises and someone gets offended. I have seen people get upset because they couldn't find their glove, got in the water late and complained that they didn't get a fair chance. Think of possible situations and try and be prepared for them. If everyone has a good time then all is well, but, if a problem arises that you were prepared for then you are the hero (even if your the only one that knows).
