Through the island channel out to the open sea they went, all except the steersman hanging over the side of the craft and enjoying the amazing sights in the clear depths below.  Bob excitedly pointed out a group of six or eight big tarpon lazily wallowing about fifty feet beneath them.  And less than two minutes afterward, Paul, in no less excitement, announced the discovery on his side of a big nurse-shark which was rolling an eye at him from the ocean's floor.  John pointed out, from the bow, a great school of fish numbering possibly ten thousand, which Mr. Choate stated were small mangrove-snappers.  They were parading up and down a stretch of coral shelf along the bottom, and they made a wild dash and hid in crannies under the coral as a big barracuda unexpectedly shot into their midst and grabbed one unlucky snapper.