Getting any weapon from the seachests is randomized, some people might get it with the first fish they catch and others might be unlucky and get none with a million caught fish
Kinda took me around 40-60 sea chests to get the Kraken. If you're trying to get the Kraken out of determination, you'll get bored of fishing fast. Just keep fishing while enjoying it and the Kraken will eventually come to you.