At least that's often spelled "fo'c'sle" these days, which gives you a good idea of the actual pronunciation.
My personal favorite in English is "colonel" being pronounced the same as "kernel". Which is insane even from an etymological perspective because the word is a derivative of "column" (as in, a colonel is someone who commands/leads a column of soldiers).
A lot of nautical terms have unusual pronunciations. English sailors primarily came from coastal regions, and were very happy to have a lingo that was incomprehensible to the landsman. All of this carried over to North America as well.