great advise from cali! If you find a ships excursion that you really want to do, then book it on-line in advance,if you can - one less thing to worry about, one less expense on your sign and sail card. If its a really popular tour it could sell-out. If you are booking your tours with an independant tour company then by all means you should book them in advance.
Tell us your ports of call and what your interest are and we can make some suggestions for you.