No coral heads in that area - just turtle grass, sugar white sand, and some chunks of hard substrate in a bit deeper water, but overall, fairly soft bottom. Tarpon tend to forage at the particular location because of it's proximity to the cuts just above Stock Island, which during ebb tide wash out dollar crabs and a host of forage species. It's also a great place to hook up with grunt, which is among the best tasting fish in Florida. The grunt hold along the deeper edges of the cuts and feed on baby shrimp as the tide ebbs and carries them out of the turtle grass.
As for the GPS accuracy in that area, it's actually very, very good, though I'm sure the charts in the GPS are quite dated, probably back to the late 1940s or 50s. Many years ago, when Key West was still pumping out it's treated wastewater into the Ocean, there was an underwater discharge pipe about 1/4 mile from the beach that locals referred to as the Rose Bowel because of it's odoriferous waft when the wind was blowing from the south east. My GPS location of the Rose Bowel was within 20 feet of the actual pipe, which I consider pretty accurate. Also, the GPS found the day markers that marked the entrance to Mule Key Basin and the accuracy there was just a couple feet at most.
I'm fairly confident that the line drawn by the OP was on the money, and I'm also confident that recent storms could have easily washed that sand bar away, thus providing him with deep water to transit the area. The same thing happened at Boca Grande Key, where a popular sailboat anchorage can be found at the end of the Lakes Passage. At one time, back in the 1970s, it was very unusual to see a sailboat at that locations because the entrance had silted in to about 3 feet at high tide. Today, that same entrance is now about 8 feet deep and lots of sailboats utilize the anchorage area on a regular basis. That depth change was attributed to the passage of a powerful storm more than 2 decades ago. Unfortunately, I have not visited this location in more than a decade, and from the Google Earth photo it appears to have filled in once again, though because the water is gin clear in this location, it is difficult to tell.
If you happen to get to the Boca Grande Cut Anchorage, the tide screams through the cut during ebb tide but the bottom holding is pretty good because of the hard substrate. And, when the tide slacks, it's a fantastic location to catch lobster, huge grunts, mangrove snapper, small grouper, and cero mackerel. This location has provided me and others with lots of fantastic seafood dinners.