Guided sea cave trips run at times that appear arbitrary until you understand tides. The window is dictated by the height of the cave floor and how fast the water is rising.
The cave is a fixed elevation and the sea is not
A cave entrance sits at a fixed height above chart datum. The sea rises and falls past it twice a day, so the entrance is submerged, passable or dry depending on the hour.
The usable window is the period when there is enough water to float a boat in and enough headroom to pass under the entrance. Those two conditions can conflict.
Where the entrance is low, the window may be only an hour either side of low water, and on a large tidal range it can vanish entirely on some days.
Tidal range varies through the month
Tidal range is not constant. It cycles roughly every two weeks between larger and smaller ranges as the sun and moon align or oppose, which changes the height of both high and low water.
A cave accessible at low water on a large range may not be accessible at all on a small one, because low water simply does not fall far enough.
Operators plan seasons around this cycle, which is why cave trips cluster on particular days rather than running every afternoon.
The tide does not move at a constant rate
Water level changes slowly around high and low water and much faster in the middle of the cycle. A window that looked comfortable can close quickly during the fast part.
Guides plan for the exit rather than the entry, since being inside with a rising tide and a shrinking entrance is the hazard the schedule exists to avoid.
Swell matters more than tide height
A modest swell that is unremarkable on open water becomes dangerous in a confined entrance, where wave energy is concentrated and the water surges vertically against rock.
Long-period swell from a distant storm can arrive under a clear sky with no local wind, which is why trips are cancelled on days that look ideal from the beach.
Why this is guided work
Combining tide tables, swell forecasts and local knowledge of a specific cave's floor and entrance is not something a visitor can assemble from public forecasts alone.
Sea caves also have no exit other than the one you entered, and conditions inside cannot be assessed from outside, which removes the usual option of turning back partway.
Anyone considering independent access should treat official coastal safety guidance and local rescue advice as the governing source rather than a general description of the mechanism.