Steve, thanks for thinking about that and posting. We're not going toward the mouth, we're going "upriver." As Michael mentioned, neither of us has kayaked the north side of the island, but I paddled there with the Lumpy Water folks on the south side of the island with an almost identical tide situation. Michael visited and took pictures on another similar tide day, though he didn't kayak that day. We agree 100% that you have to be careful about the tides, and we have given that thought and are addressing it.
I have paddled Sand Lake and watched it ebb & flood from Whalen Island. I believe that your proposed trip is ill advised. Sand lake is fairly large and it becomes too shallow for kayaks at low tide. During ebb a large volume of water rushes out to sea thru a small gap in the dunes creating strong suction currents. If you paddle out near the mouth and then have a leisurely lunch when you relaunch you will be in danger of being pulled out to sea by a rip tide and/or of being stuck miles from the launch with no navigable route back to your vehicles.
Oregon Ocean Paddling SocietyPortland, Oregon