Do we need to book a shore excursion for every port??
What you do in each port is completely up to you, excursions or not. You can even stay on the ship if you please and enjoy crowd free facilities. Or you can get off, walk around, and come back to the ship for lunch. Bottom line...come and go as you please and do as little or as much as you like.
how do we find something to do/ what is available?
There is much information on the internet by googling each islands tourism site and just start looking for things that interest you....beaches, cab fares, points of interest, maps, etc. it's all there. Many times something like an island tour can be booked privately for much less than the ship sponsored trip, especially when you are talking about every $ X6, yes that would add up!!
Also TripAdvisor is a great resource for information, attractions and activities are rated by users thru reviews and photos.
Good luck, I hope you find fun things to do!
"Life has no limitations, except the ones you make."