Water is the worst place to be during an electric storm. A swimmer is the highest object on a flat lake or open sea, and water conducts the ground current from a nearby strike across a wide radius — you don't need to be hit directly to be killed. The strike distance that matters is 10 kilometres. If thunder reaches you within 30 seconds of a flash, the storm is within 10 km and you must exit immediately. A storm 10 km away is moving, and at typical thunderstorm speeds of 30–50 km/h it can be overhead in 10 minutes.
The rule is called the 30-30 rule: count the seconds between flash and thunder — if it's 30 or fewer, get out. Then wait 30 minutes after the last thunder before re-entering. Most lightning-related deaths in open water happen because swimmers used their own judgment about "it's passing" and went back in too soon. The 30-minute window is not conservative padding; it accounts for the trailing edge of the storm cell, which produces lightning well after the main body has cleared.
Don't rely on the sky alone. Check a lightning forecast app before you leave home — Ventusky, Windy, and the UK Met Office's "lightning" layer all show real-time strike locations updated every few minutes. If there are strikes within 20 km of your planned spot, move your swim or pick an indoor session. Morning slots in summer are consistently lower-risk than afternoons, when solar heating builds the convective cells that produce thunderstorms across central Europe and the UK. A 6am swim in July is rarely interrupted by lightning; a 4pm swim is not a reliable bet.
Once you're out of the water, don't shelter under a tree. Trees are taller than you and are struck far more often than open ground. Move away from the water's edge — a strike on the surface creates ground current that radiates outward — and find a low, open area or a hard-roofed building or car. If none is available and you're caught in the open, crouch low on the balls of your feet, minimise your contact with the ground, and cover your ears. Do not lie flat. A crouch keeps you low but limits the ground contact that transmits current.
Group swims add social pressure that makes leaving harder. Someone always says "it's fine, look, it's miles away" — and they're often right, until the one time they're not. If you're leading or organising a group swim and you hear thunder, you call the session. That's it. No vote, no consensus. One death in a group swim that could have been prevented by leaving earlier ends the group, and frequently the sport, for everyone involved. If you're a participant and the organiser doesn't call it, you are entitled — and correct — to leave on your own.
The one specific thing to do before every summer swim: open a lightning radar on your phone, check the 2-hour forecast, note the closest active cells and their direction of travel. It takes 90 seconds. Combined with the 30-30 rule in the water and the 30-minute wait before re-entry, it closes almost every gap that electric storms open. The rule is simple; the discipline is following it even when the sky looks mostly fine.