When to go rockpooling in Britain
Why British tide tables have no negative numbers, when the biggest spring tides of the year arrive, and what a British shore holds that a Pacific one does not.
Read the number differently
A British tide table will not show you a negative low, and you should not go looking for one. Predictions are measured from near the lowest astronomical tide, so almost every low water is a small positive number. A low of 0.9 m is unremarkable. A low of 0.3 m is excellent, and it is the same shore-clearing event an American would recognize as a good minus tide.
Equinox springs, and your harbor’s hour
The biggest tides of the British year come within a fortnight or so of the March and September equinoxes. Those are the days to plan around, and around the south-west in particular the difference between an equinoctial spring and an ordinary one is a different shore entirely.
The useful local fact is that at any given harbor, spring low water falls at approximately the same time of day every fortnight, because the lag between the moon and the tide is a property of the place. Which is why people who know one shore know its hour without looking it up, and why looking it up once for your own shore is worth more than any general rule.
The season is late winter to early summer
Britain is not a summer-only proposition. Late winter and spring bring the equinoctial springs, the water is at its clearest before the summer algal growth takes off, and the shore is emptier. March to June is the sweet spot; a bright still day in February is better than a warm one in August.
The trade is temperature. Sea temperatures round the British Isles run from about 43°F (6–10°C) in late winter, which is well inside cold-shock territory, so this is a boots-and-fleece activity rather than a barefoot one.
What is on a British shore
A fauna that rewards close looking more than it rewards spectacle: beadlet and snakelocks anemones, common and cushion stars, shore and velvet swimming crabs, dog whelks drilling mussels, blennies and gobies in the deeper rock pools, and a nudibranch list far longer than most visitors suspect.
Cornwall, Pembrokeshire, the Gower, the Northumberland coast and the west of Scotland are the classic ground. The Wildlife Trusts run recording surveys on many of these shores and welcome records, which is a good way to make an afternoon count for something.