The first proper coastal walking trip I did, I planned around the weather forecast and the opening times of a couple of places I wanted to see. It went badly in a way I did not anticipate and could have avoided entirely with a table that is published free everywhere.
Two of the beaches I had planned to walk along did not exist at the times I arrived. Not metaphorically. The water was against the cliff.
How much the water actually moves
This is the part people from inland places consistently underestimate, and the variation between locations is enormous.
In some places the difference between high and low water is under a metre and the beach barely changes. In others it is more than ten metres of vertical movement, which translates into hundreds of metres of horizontal beach appearing and disappearing twice a day.
A bay that is a wide expanse of sand at one hour is entirely underwater six hours later. The photographs you saw were taken at one state of tide and tell you nothing about the other.
I had assumed a beach was a place. It is better understood as a state that a place is in for part of the day.
The rhythm, roughly
Two high tides and two low tides in a bit over twenty four hours, in most places. Which means each one arrives roughly fifty minutes later each day.
That fifty minute shift is the practically important bit. If low water is convenient on the day you arrive, it will be an hour later tomorrow and by the end of the week it is in the middle of the night.
Any plan that depends on low water needs to account for that drift, and a week in one place will only give you a few days where the timing works.
There is also a monthly cycle. Around new and full moon the range is largest, and in between it is smallest. A spring tide exposes far more beach and comes in far faster than a neap tide.
What this changes about planning
I now look at the tide table before I book anything coastal, and it has reshaped several trips.
Anything involving walking along a beach between two points needs a falling or low tide, and needs to be finished before the water returns.
Rock pools are a low water activity and are simply not there otherwise.
Any causeway or tidal island has a crossing window that is published and is not negotiable. People get caught by these every year and coastguards spend a lot of time on it.
Swimming is affected too. Currents around a headland are strongest during the middle of a tide when the water is moving fastest, and slackest at high and low water. Local advice about when to swim is almost always about this.
The safety part, which is the serious bit
The specific danger is being cut off, and it happens in a predictable way.
You walk along a beach at low water, round a headland, and continue. The tide turns. The water reaches the base of the cliff at the headland behind you before it reaches where you are standing.
Now you cannot go back, and forward may not lead anywhere. This is the situation that produces most coastal rescues and it is entirely a planning failure.
The rule I follow, having read too many accounts of this, is never to walk past a point I could not get back around, and to turn around no later than an hour after low water rather than working out precisely how long I have.
Rising water also moves faster than people expect across flat sand. On a wide shallow beach it can advance at walking pace or faster, which is why people get caught on sandbanks.
Where to get the information
Tide tables are published for every significant coastal location, free, by national hydrographic services and by countless apps.
The thing to check is that you have the table for the right place. Times can vary by an hour or more between locations a short distance apart, depending on the shape of the coast, and using a table for the nearest big port can be badly wrong in an estuary.
Most coastal towns post the day's times somewhere public — a harbour office, a lifeguard station, a noticeboard. Photographing that on arrival takes ten seconds.
The upside, which is the real point
All of this sounds like constraint and it is mostly opportunity.
Low water on a big spring tide reveals things that are visible a handful of times a year. Reefs, wrecks, caves, whole stretches of shore that are normally underwater.
Planning a trip around a big low tide, rather than around the weather, gets you a coastline that most visitors never see, and it costs nothing but looking at a table.
The best day I have had on a coast was a spring low on a calm morning, walking on a seabed, and I only knew to be there because I had finally learned to check.