I suspect I will still need to check the cup more often than when not diving as my flow could be heavier than I might expect but I'd hope it can contain/hold more blood than the tampon could hold and not result in major leaks.
The only thing I may need to be concerned about is having bloody finger(s) but there should be water nearby to go rinse my hands!!!...I'd rather have a bloody finger than blood soaked bathing suit/wetsuit!!