A 27' Hunter would be an ideal boat for going down the back side of the Keys, which I think is a wonderful trip... No need to go as far as Marathon, cut across Florida Bay via the Yacht Channel, just past Lower Matecumbe...
I've done this trip both on the Okeechobee and via the Keys - the above comment is the way to do it. Controlling depths are found just west of Islamorada, at a little over five feet- and I may have been on the edge of the channel since depths are shown greater than that. Make sure you stop at Little Shark River, wonderful anchorage.
The Okeechobee is alright, esp with a boat that size, but given you're in Biscayne Bay, why would you want to do that to yourself?