What's the Name of That Book??? discussion

I read the book about 30 years ago. I read it in Swedish. I remember two boys being the main caracters. They were english/brittish. Somehow they end up fighting underwater pirates, struggling to keep them from cutting knots/ lines that hold the world together. I remember that they were able to stay underwater by using some stinky blubber etc. One of the bad guys were names something along the lines of Nincompoop. They were able to ride various wales, Sunwales etc. The setting felt like it was prior to 1930, but I could be wrong on that. I think the boys had an uncle that had lost one leg, an arm and an eye in a seabattle. The uncle was not under the sea. I remember reading this book with a lot of joy but cannot remember the title or the author. I have tried every possible search parameters I could think of. If it helps, the book was a paperback and I remember it as being quite a long read.

The pirates in the deep green sea, by Eric Linklater, Cully the octopus appears.

Thank you M! I'm ordering it. My kids will love it.

The Pirates In The Deep Green Sea by Eric Linklater for the link.