Kurt, Steve, and I visited s7 and added an external cell antenna with a small piece of angle iron as the ground plane. It actually came back online just as we got there, before we attached the antenna. I logged into the DSM with the Acer and browsed to the cell modem web interface, but I was unable to check the signal level before and after attaching the antenna because the left mouse button on the touchpad is broken. We at least verified it came up after plugging the modem back in.
We visited S10 and attached a cell antenna there also. Kurt powered it down briefly to adjust some wiring. Then we verified that Internet and data stats were working. The antenna cable at S10 has exposed wire, so it needs to be wrapped with some black tape at some point. Right now the bad spots are coiled up and ready to be sealed in tape.
Steve did most of the boat operation and more of the navigating, as Kurt tries to teach us the lay of the marsh. We went the long way both in and out. We boated passed S3, but the stake says S1, so that needs to be fixed.
Steve and I have many of the sites marked on our phones, but since google maps does not show all the channels that are needed to get to all the sites, I would like to find an accurate paper navigation chart or topo map. Also I bet there are better phone apps for charting waterways, but I haven't looked.
The cell antenna ground planes are just small pieces of iron scrounged up in the trailer. We also considered using the metal bookends. If any other sites have trouble connecting, it might be worth buying small pieces of sheet metal to give the antennas a proper ground plane, but I'm not an RF engineer so I'm just guessing. 
A few other sites already have antennas that were mounted without ground planes.