On the Marigot ferry - sometimes you cannot stand on deck, you have to sit below
If that happens, our experience is the spray from the waves blows from the north, so when going to Anguilla, the starboard [right facing forward] ports are closed and the port side [left facing forward] ports are open
So if queasy is an issue sit on the port side [left] to get some air

Reverse sides for the trip back to SXM - but in case on deck is the best option if possible. Short trip, 20+ minutes

Remember the ferry is cash
Enjoy
J&B