It's fun! I like it
A few thoughts:
1. In the second line, do you mean 'rises', and in the third do you mean 'strike fear in'?
2. I don't quite understand this line: At the end, they carve for the sea (but I know little of pirating!)
3. It might just be that I'm not reading it in my head with the rhythm you intended, but the two really long lines seem to break the flow a bit for me. Particularly, this one:
For sometimes they are gentle at times, but only when they drink, drink ale.
would read better for me if it said "For they are gentle at times, but only when they drink, drink ale".