All - many thanks for the great inputs - much appreciated.

I checked with Native Son and Aquatic Rentals, and neither offers cancellation insurance, though Native Son did say that they will offer a refund in the case of a flight delay. On the Walk-ups (i.e. no reservation), Native Son's website is silent on the topic, but Aquatic rentals clearly says that they will take these (up to 30 mins before departure).

Anyway, our plan is to book the 5pm Native Son ferry.

Many thanks!