Here in the NE we wouldn't call five feet deep draft, we'd call it shoal.(G)
A peek at some online tidal sites says right now "sea level" is about two feet over mean low water in the Keys (with a tidal variation of less than a foot) so if the chart is drawn at mean low water, and right now (moon phase, winds, rain runoff) there's another two feet on top of that, the water depth could be four feet despite the chart.
Assuming the chart is recent, not 1840's. What's the date on it? (Yeah, electronics tend to omit that when they tile charts.) And accurate or certain was your position on the chart, and the position data used to mark that spot?
Charts at Jones Beach (LI, NY) twenty years ago were considered accurate and well accepted--but sometimes were off 1/3 mile compared to GPS.
And then there's the fact that the oceans are living systems, not static. Channels routinely scour out, I've been in a "forty foot" channel and measured over eighty feet to the bottom, courtesy of current scour. Of course if the coral was healthy, the bottom could go the other way as well.
Which is why I'm semi-serious when someone asks me about sufficient depth, and I say, if we don't have enough water to roll the boat without getting the mast stuck in the bottom, we don't have enough water.
If you've ever seen a shopping cart (?!), oil drum, or trashed 16' runabout sitting in ten feet of water and cutting it down to a six foot clearance...Yeah, the Keys and coral heads and all that fine stuff are very different from northern waters and rock ledges.(G)
BTW, NOAA still appreciates chart correction data and survey data being sent in from all vessels, so they can fix these things.