For roughly six hours the tide will take the swimmer 'up' the Channel, and then as the tide changes direction, the following six hours will take the swimmer 'down' the Channel. This up and down movement of the water is relentless and unavoidable.

When traversing the English Channel, the boat pilot pays respect to the aformentioned tides when heading for France, which means the tidal affect will be perpendicular to the direction of the swimmer. It is incredibly rare for a swimmer to ever be swimming with or against the tide.

The moon's position relative to the earth and sun changes, creating different strengths of tide. The smaller tides are called neap tides, and the bigger ones are spring tides. Historically, swimmers have made their attempts on neap tides, as the belief is that this reduces the effect of wind against tide. It also reduces the risk of the swimmer missing the land target of Cap Gris Nez in France.

Source: Channel Swimming & Piloting Federation