Short answer, there is no single profit stream for selling trips.
Selling extra gear is a good one. Been holding out buying something, better get it before the trip.
Sometimes they get a bulk rate on buying a whole trip, thus room for markup.
Or get a comped seat on the boat, etc. This is often getting a free vacation for the work of putting everything together. And not really free, just heavy discount when you consider all the other costs.
Add in some after trip sales. Man these fins really sucked on that last trip, got anything better? I need a better computer. I had the weakest light on the whole boat, sell me one that won't embarrass me on the next trip.