Never really understood why people put a "t" sound into the word when it's not present in the word. I also don't quite under why the "CH" that has it's own Cho-colate and Ch-aracter ish style to it would become a "k" sound. Of course I'm not a great English major so maybe there's some logic behind why it would become such, but plainly looking on the word itself it has always been a.
Fi-sho or Fee-sho pronunciation to me, or however you want to explain that fishy sound. Seems the most logical pronunciation to me.