I guess not knowing squat about which bone you broke, if it is a simple fracture, or if it is comminuted (shattered in multiple pieces) - it would be nearly impossible to guess the duration of your restrictions. Theoretically, you could dive with only one arm. But what you intend to do with that arm while diving, we'll never know - and that is where the restriction needs to be imposed. Usually, a good scar tissue is formed in 4 to 8 weeks, and will give strength to the bone. Kids tends to grow the callous faster, and old folks tends to grow it slower. They might xray it at 4 to 8 weeks to verify the normal healing process, and will clear you for activities at that time. If a non-union occur, you might need a second surgery to reposition the bones or they might add a bone graft. At least this was vaguely what I remembered from vet school, he, he, he... I am sure you're part primate too.
So I would say, it is up to your orthopedist. As I understand, a bone plate straddled across a bone does not make it stronger than before. Actually, by bearing the weight or stress of the bone, it actually make the bone weaker. So be careful, and don't think that you are a bionic man with a stronger arm than previously.