"anyone have advice or stories about using the scuba gear provided by the cruise ship?"
I don’t know of any cruise lines that provide gear, if you book a dive thru them it will actually be with a local shop and having seen some of the gear used in third world countries I would lug my own. It’s bad enough that the tank on your back may never have been hydro’d and the O-ring is suspect!
One more caution, when returning to the ship you may need to go thru that countries version of TSA, last year returning from a dive we had a run in with the Greeks TSA, a dive knife is a weapon!