I once heard a BBC weather forecaster explain how to plan a weekend outing, while maximising the likelihood of good weather.
It amounted to the following: If Saturday is nice, then go. If it isn't, then don't go. But you will have to go on Sunday, then, whatever.
I think that strategy may not be applicable to sea crossings, but I do have this tip for minimising seasickness. Remember to find somewhere where you can see the horizon, and keep looking at it. This gives your brain vital information about what the underlying level playing field is, IYSWIM, or you could say rather, what a level sailing sea would be i.e. flat calm. This helps your brain process the fact that the sea is far from calm atm
.