Look into the guides that come out of Hawk's Key, they are great.
I can't recommend the one guy my dad has used for the last several years due to a massave stroke. But he always works with the other guys that guide out of Hawk's Kay.
It's a big resort that is located just north of Marathon, plus it seems as if the bridges right around in the area hold lots of tarpon. We saw them swimming all around the docks there too.
Good luck and hope you have a blast.