It really depends on what you are going to be doing. If you are taking a ships excrusion to just go to a beach for the day, and there are four of you, then it would be considerably less money to just grab a cab and go to the beach.

If however you are going to visit a very popular site (though I can't think of an example of this in the Islands, but if in St Petersburg, say the Hermitage), then the ships tours may get you around the long lines and the rest.

There are places where the cruise line vitrually books everybody as marysunshine indicates, some of the fishing trips for example are under contract when a cruise ship is in town, etc. But for the most part, if it's just a trip to this attraction or that one, and it's not peak season, then you can do them on your own. You don't, for example, need a shopping excrusion, if the shopping area is within walking distance of the ship.