The Coral Island / The Young Fur Traders R. M. Ballantyne

When the three sailor lads, Ralph, Jack and Peterkin are cast ashore after the storm, their first task is to find out whether the island is inhabited. Their next task is to find a way of staying alive. They go hunting and learn to fish, explore underwater caves and build boats - but then their island paradise is rudely disturbed by the arrival of pirates.