All the passengers were seasick during the storm.

Literal

Storm [possessive-の] during [topic-は] ship-passengers [topic-は] all seasick-did.

船客 (ship passengers) is a compound: 船 (ship) + 客 (passenger/guest). 船酔いする (to get seasick) = 船 (ship) + 酔い (drunkenness/sickness). みな (all/everyone) emphasizes that nobody was spared.