Heck, when there were 4 of us, we booked rental cars at most ports through a major car rental agency (Hertz, Avis, National, etc.) and were able to cover what the tours did much faster or more thoroughly and much, much less expensively.

But we did our homework and had tour books and maps and reservations all taken care of in advance (plus we had worked out how to get to and from the rental car office in advance in most cases).

The price markups on those on-board excursions are HUGE.

Depends on your self-confidence and the homework you do.

Also, never, ever believe a store recommendation from an onboard port lecturer as every recommendation is a paid recommendation. Not just 98% of them, not even 99% of them, but all of them.