And if I'm attacking ships, how would I do this without a cannon?
Ramming was a big element of pre-gunpowder naval warfare. Ram, board, murder, take, and repeat.
Why do you believe it would be easier to raid costal communities? In its early history, the republic of Venice seized various cities along the Dalmatian coast (Pola, Zara, Ragusa, Spalato) to better check pirate raiding in the Adriatic sea and give ships safe havens en route to Venice.