I've heard they were rare, like one fish cave per 50 ... but in my own two games played on mistlands patch, i had at least one in first 5 caves. Its probably lucky roll thou... after like 30 caves total im still finding new things, like frozen lake. There is a lot of content for the caves.

Dont get your hopes up thou, the cave pond is quite small and the fish jump out on their own, never had to fish for them...