"according to the US Census Bureau, it ends at land's edge, "
Understand that the key criteria behind all USCB actions is enumerating voters.
I had some discussion with them regarding the popular free TIGER maps that so many people relied on before Google Maps existed. Discussing the inaccuracies (often a mile or two) and the way roads and districts were drawn, they said, well, sometimes a new development or street was built and someone walked or drove down and said "yeah, that's about 1/4 mile, west by northwest" and that's all THEY needed to determine who lived in which voting district.
Their mapping was never intended for navigation or other purposes, just for census enumeration.
Now since no one can have a residential address in a municipality unless it is recognized by the USPS, and those are all ON LAND...you can see why the USCB really doesn't concern itself with what happens in or underwater.
Unless you're going to build a habitat anchored on the floor of Biscayne Bay, and then demand municipal trash collection and mail delivery. And pay real estate and sewer taxes on it.
Not their concern.
If you want the real facts? Somewhere, a municipal or corporate charter was filed with the State of Florida, for your municipality. In fact, the locals most likely have a Zoning, Building, or similar department, which actually has engineering plats (detailed topo & street maps) or at least maps of TAX PLOTS and building plots for the whole "city". They will clearly (and perhaps accurately) show the designated city limits, to make sure every cent that can be collected, is being collected, right up to the "town" line.
Got a "town hall" or "city hall" with a clerk in it?