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.