The most important thing about landing below the high tide mark, is to remember to only do it at LOW TIDE.
In PNG in the 70s with a Huey, had to pass a message to the chief of a tiny island, so we landed on the beach. While the Boss was away dealing with the Bospela, i pulled the starter circuit breaker.
My boss returns after a while, notes the imminent arrival of the next tide and rapidly straps in. Hits the tit, nothing happens. You should have seen how low his jaw went as he considered the possibility of submerging the aircraft.
Crewman and me rolling in our seats, reset the CB and spent the rest of the day in deep 5h1t from the boss.