Based upon my experience as both a SCUBA diver, and longtime bullet caster I can tell you that diving weights are made out of whatever was laying around. I have some I’ve acquired over the years that are good lead, others that are some non-specific alloy, most likely wheel weights.
If they “clink” they aren’t pure lead may be too hard to use for patched roundball shooting. Good soft lead will “ thud”.
Of course, you can always give them a try.
Some folks find wheel weights work just fine.