Water Taxi...

I have a water taxi this morning, delivering 18 people to another island.  Assuming they are on time (arriving by ferry and sea plane), I should have just enough time to get them to their destination, turn around, and make it back just in time for the scheduled whale watch trip.  Assuming we get out of there on time, I should have just enough time to get back, and run back to the first destination this morning to get those people back just in time for sea plane and ferry departures.

What could possibly go wrong with that plan? 

(Insert your favorite "assume" definition here)