As it name indicates, the sea mill - or tide mill - is operated by the tides, but not, as is commonly assumed, by the direct action of the ebb & flow.
Its operation is actually the same as for any water-mill once the sea has gone out far enough to leave the wheel dry - the force of inertia exerted by water would otherwise prevent it from turning. In the case of the Birlot mill, this corresponds to half-tide, which means that the wheel can be operated for 6 hours : the last 3 hours of ebbing & the first 3 hours of rising-tide.
Of course, if need be, the miller can work it twice a day, since there are 2 tides in 24 hours.
The Birlot mill was once used to extract flour from wheat, barley & buckwheat.

With the incoming tide water gets into the sea-pond through the sluice gates.
The wheel can't turn.

With the ebbing tide the sluice-gates swing shut.
The water kept in the sea-pond will supply the energy needed to work the mill.
The wheel can move only when the sea has ebbed far enough.
The miller then lifts the spade : like hatch thanks to a lever to let the water out into the mill-race. The wheel starts revolving, to stop only when the flow comes back.