You cannot “just” cross a sandbar.

Tennyson summered on the Isle of Wight for more than 20 years during his time as Poet Laureate. In that time, he became familiar with nautical realities of navigating the English Channel in the narrow Solent Strait where the deep and turbulent waters of the open channel came into the inner harbor network of the island.

To use a boat crossing over one of these sandbars as a symbol for dying suggests that Tennyson is not pretending that the “crossing” itself will not be without its terrors. Experienced sailors understand the tricky and sometimes treacherous reality of attempting to push a boat across the sandbar. Incoming deep water brings with it huge deposits of sand which, when the waves hit the shallower water of the strait, get dropped along a line roughly separating the strait from the open sea. Those sandbars are mapped to help in navigation but the nature of the currents can alter the position and height of the sandbar. No sandbar is the same. Today, any boat attempting to cross the bar at Solent Strait must register before attempting the crossing to help in any search and rescue operations should the crossing go badly.