This had to be one of the most startling things I've ever seen.
Twitter user Kaydi_K, a resident of Long Island, Bahamas, took a video off a dock at the shore of her home, showing absolutely no water where the ocean was only a day before, as far as the eye can see.
Meteorologically, Hurricane Irma is so strong that the low pressure at the center caused all the water to get sucked up into itself. Meteorologist Wayne Neely explains:
[T]his by the way is not a normal phenomena but has happened several times before but it is due to the hurricane. What happened in this case was that the low pressure of the hurricane pushed down on the water in the eye and eye wall of the hurricane which is a low pressure system at at the center of the storm which gives you the high level storm surge but in nature the is a never ending process of trying or forcing to maintain a perfect because at the center the water height is pretty high and Irma had 15-20 ft storm surge but that water which produces this bulging effect takes water from its surroundings to be transferred into the eye so the water receding is part of this storm. But in the Long Island and Exuma cases care must be taken in this case because the water often returns with even greater fury.
It's important to distinguish that is not the same as a tsunami and the water is not going to come rushing back in some giant wave.
However, there's no escaping that a significant portion of that water is now barrelling to the Florida coast.