Certifications of vessels are great, but nobody should take a sticker (whether it's the USCG or some other outfit) as evidence of a boat's seaworthiness. All the sticker means is that the boat passed muster sometime within the last year, probably on a date when the captain knew the vessel would be checked. Many boat owners only have the required equipment onboard at the time of certification. After that, they may not have them onboard (or operational) until their next certification a year later. (And that's in countries where there are standards and methods of enforcing them!) When I was a fisheries observer, I had to check for about 18 different things on every boat every time I sailed with someone—even if I'd been with them the day before—precisely because the captain might have loaned things like safety suits, rafts, life jackets, and so on, to someone who was having a USCG certification that day, or someone who'd already been busted for not having everything he was supposed to have when they got randomly boarded out at sea. And even if everything is in place, you can't assume each item is still operational or reliable. Check the dates of their last tests, have the captain tell you about raft deployment, EPIRB operation, fire extinguishers, life jackets, and so on. It's one thing to have the stuff strapped to the wheelhouse, and it's another to have stuff that works.
Divers and travelers should develop their own standards and ask the vessel ownership about safety equipment before booking the trip. If that's not doable, just ask to see the boat before you pay, or before you depart. If something isn't to your liking, find another boat that doesn't have a life raft that's 30 years old or an EPIRB that hasn't been tested since 1989.
I'm glad this topic has been raised. Too often, especially in warm latitudes, we just hop on boats without checking this stuff out at all.