I've been fishing for a couple days, and I'm wondering if I may be wasting my time. I can't find an answer where I look so if you've found one by fishing or ship loot, please let me know!
The Wiki says that you can rarely obtain it in both fishing and sailing loot; I've never seen it or heard of it being found in either of those so I'm going to say no, you can only find it in Cannon Defense loot.