I could be wrong, and usually I am, but I do not recall snorkel equipment rental being an option offered on my last Carnival cruise in May. They would sell you a snorkel for $5
since many folks are skeeved about using a used snorkel on excursions. Most excursion operaters (whether ship booked or independent) will supply equipment
and many supply a certain amount of prescription masks these days.
I sugeest , if you like snorkelling, that you buy and bring your own stuff. This way there are no sanitary issues, no issues with prescription masks etc.