Boarding the boat from Nusa Lembongan back to the main island of Bali.
If you get seasick on occasion I would recommend taking something before getting on the boat. While the waters are normally calm, it can get bumpy and you are inside a boat as there is no seating on deck.
I would book in advance especially if you are travelling during a busy season. The boats we took were fully booked both ways. Also, if you want to take advantage of the free transportation it must be coordinated in advance.