On many coastlines the tide determines what is possible on a given afternoon more than the weather does. Understanding the pattern changes how a coastal day is planned.
The cycle advances through the day
Most coasts experience two high and two low waters daily, with each successive tide arriving roughly fifty minutes later than the equivalent one before.
A beach that was wide and dry at three o'clock will be at a different state at the same hour tomorrow, and noticeably different within a week.
This drift is why local advice given on arrival becomes wrong quickly, and why tide tables are consulted rather than remembered.
Range varies through the month
The difference between high and low water is not constant. It widens around new and full moons and narrows in between, following a fortnightly pattern.
During the widest tides, low water exposes ground that is normally submerged and high water covers areas that usually stay dry.
These periods produce both the best rock pooling and the most frequent problems with cut-off beaches, which are two sides of the same range.
Access routes appear and close
Causeways to tidal islands, coastal paths beneath cliffs and routes around headlands are usable only within a window either side of low water.
The closing edge matters more than the opening one, since the risk is being on the far side when water returns to a route that has no alternative.
Published safe crossing times are usually conservative, and they account for the speed at which water fills a channel rather than simply the depth.
Water clarity and current follow the same cycle
Incoming tides typically bring clearer water from offshore, while outgoing tides carry sediment from estuaries and lagoons, which affects visibility for snorkelling.
Current is strongest during mid-tide when the volume of water is moving fastest, and weakest near the turn at high and low water.
Dive and snorkel operators schedule around this, which is why departure times shift daily and appear inconsistent when compared across a week.
Boats and harbours run to the same clock
Shallow harbours and moorings dry out at low water, so small craft can only leave and return within a defined period each cycle.
Trips are therefore timed to the tide rather than to convenience, and a booking made for a fixed hour may be moved once the date approaches.
Treating the tide table as the first item in a coastal itinerary rather than a detail resolves most of the frustration these constraints cause.