its really up to you. If you find an excursion thru the cruise line that you think that you would really like to do i would book it now. The really popular excursions book up fast and can sell-out. You can always cancel once onboard up to 24 hrs before your excursion. I am not a huge fan of booking an excursion from the hawkers at the dock, you just don't know what you are getting yourself into.
FYI - if you find an independent tour operator on line, post it here and ask for feedback