Mystery bird!

On this morning's dog walk around Mason Wetlands I watched a small heron-type bird flying around in the distance.  It would fly east, then back west, at some point dive straight down, then fly back up and east till it eventually disappeared.  The three options that came to mind were Green Heron, American Bittern, and Black-crowned Night-Heron.  All are possible there. 

Looking at my photos I eliminated Night-Heron because of the bill shape/length.  What do you think?

Also, what was it doing with all the diving and whatnot?   Thoughts please!