About the tides in the Thames estuary.

The river Thames at 215 miles (346 km) is the longest river in England. It flows from its source in Kemble in Gloucestershire to its mouth in Essex where it joins the North Sea. Its first 147 miles (237 km) to Teddington lock are non tidal. Below Teddington lock the tides from the North Sea take effect and the next 68 miles (109 km) have constantly rising and falling levels as the tide comes in (floods) and goes out again (ebbs), with a high tide approx every twelve and a half hours. 
East of Leigh-on-sea (and continuing east for another 17 miles) there are extensive mudflats and sandbanks which are uncovered each day to a varying degree dependent on the tide height and wind strength & direction. Tide tables predicting tide heights are published each year for the height of every high and low tide, but remember these are just predictions and the wind strength and direction can mean these predictions can be some way out. 
The mudflats are a unique ecosystem which house a diverse range of flora and fauna so are of great interest to nature lovers. 
It is always wise to be extremely cautious when walking out from the shore. If you do not fully understand the tides and how weather conditions can affect them in the Thames estuary, please go with a knowledgable guide who does, because the water does come back extremely quickly at times, and often not from the direction you would expect it to come from, and every year people do get into serious trouble.