I've found it possible to book "excursions" based on what I want to see and do, rather than on what the ship thinks many people will want to see and do, online a few weeks ut, rather than go through the ship's purser, notably cheaper. Sometimes I have discovered it is the same tour, they pick up the group on the ship that paid too much, then pick me up a block or two away.

And, of course, doing your research and just doing your own thing in port is fine.

The sole real advantage of booking through the ship is that they guarantee to get you back aboard before the ship sails. Getting left in port is a nuisance, you'll have t catch up with the ship at the next port, at your own expense.

I've never had that happen -- just leave yourself plenty of time to get back aboard, but if I had to name an advantage to booking excursions with the ship, that would be it.

If you DO book through the purser's office, get in line and book early... popular excursions sell out early.