Snorkeling at Goff's is great. We've done it twice although not with Excalibur. However, make sure you don't go on the day that the cruise ship is in port in BC. (Wednesday I think). I heard they come with a boatload of people (literally) and commandeer the small picnic area and the leeward side beach. Not exactly my idea of a peaceful excursion. They even bring their own port-o-pottie!
There's also good snorkeling at Mexico Rocks and Tres Cocos, and a shorter boat ride than Goff's. If the tour guides aren't advertising it on their blackboards you just need to ask.