When a hurricane approaches, there is storm surge that piles up water, but when it is out at sea, especially in shallow waters, the vacuum effect of the low pressure of the storm can suck water away from the shores. Offshore winds also play a big role. Such is the case here. This photo, taken by Dana Young and posted on Twitter, shows the effect.
Hours before Irma hits, the water is literally being sucked out of Tampa Bay. I took this photo at the end of my street. This was at Albany and Bayshore 15 min ago.
Others are observing the same thing:
The same thing happened in the Bahamas when #Irma approached there:
The danger here is that once winds reverse and the eye gets closer, the water comes rushing back as dangerous storm surge.
See this graph of Sea Level from Naples: