Florida Mystery Sea Creature

When I was down in the Florida Keys back in February, I was on a beach in Marathon and saw something moving through the water that I was not able to identify. To me, it looked like a bit of seaweed, shaped a bit like a schnauzer, but its movements seemed purposeful and I watched it swim around for some time. During that, I was able to get some an okay picture and some poor video of it. Any help identifying what it is would be most appreciated.

Watch the video.

Thanks!