The math is actually more complex that that. You do not need to have a vaccine that is 100% effective to achieve herd immunity. Depending on the virus and the R0, it's usually around 70% (and may be higher or lower depending on various factors).
Remember all the things that enter in. Just because the vaccine is 90% effective, you would still have to have come in close contact with someone else who tested positive. That means you are now at 10% of 10%. plus, you had to come into contact with them for a prolonged time while they were contagious. To be contagious, they would have had to come into prolonged contact with someone else. Then you need to have the same prolonged contact with only someone else who was still vulnerable (meaning they have to be in that 10% as well) while you are still contagious.
When you add in all those scenario's, it MAY be possible that you could end up contagious with COVID and pass it on, but even then you would likely only pass it on to one or two other people on the ship if all the factors lined up just so.
That's a far cry from an outbreak. And overtime, that factor gets smaller and smaller as the number of people who are contagious that you might run into gets less and less.