I have no certificates, or references. But I'm pretty sure I would be good crew on a passage. I have experience though, mainly because I sailed my own boat on long blue water passages and in some really nasty weather. You can still find work at the dock, I was offered a job to crew a boat from Panama to China, but I didn't have the time and the pay was a little low, the skipper was nice but... intense.... He had sailed a hijacked ship out of somalia after it was captured by pirates and was an Ex eastern european soldier.
I think a lot comes down to timing. I know I'm looking for crew, but I'm asking people I know first before I scour the web. A month or two on a small boat with someone is intense, anyone can handle a few days.
Then you have not been in rough enough weather then. I don't throw up, but I get queasy/easily agitated for a few hrs when I head straight out into 25 knots and steep seas. What would scare me is crew that is too confident in their abilities, or not scared a bit about going offshore. I stay well away from people with big egos, Captain or crew.