In our group, historically if one player couldn't make it, that hadn’t been a problem and we would say that they’re sick in their room on the ship or something.
If two players were unable to make it, we might still be able to carry on, or maybe not.
But a lot depends on how many players you have.
Recently, our game was down to just two players and myself, and one of us took over as GM because the previous GM had some work issues that were conflicting. In that situation, if one player can’t make it, then there’s unlikely to be much that you can do with just one other player and the GM.
Now that we’re back to five players, one or two being out shouldn’t be a huge problem.