Continuing at same location late this afternoon (1700, 24 May 2017).
End of 13th Street in Imperial Beach. Park at lovely new Bayshore Bikeway facility at north terminus of 13th. The pond directly north of this parking is the pond, and the mudflat is over on northeast side.
You can get closer, as Paul pointed out, by hiking east down the bike path, over the first bridge. Problem is fence blocks view and nothing to step on to peer over. You can view through the fence of course, but this is climb resistant mesh.
If you keep hiking down toward the next bridge there is a section of fence you might be able to peer over, and morning light will be good. (In afternoon that spot would be looking into the sun.)