A picture tells a story

This is our river, the Great Ouse. It flows to The Wash and North Sea off to the right. But when the tide comes in the flow reverses as seawater is forced upstream to the left. At low tide the river is not navigable - water too shallow. The fishing boats have to exploit the situation and time their comings and goings according to the tide, to minimise the time battling against the flow, saving time and fuel.

The nearer boat is working hard to maintain headway, the one opposite which is returning is travelling faster with the benefit of the current. The current will soon reverse and the nearer boat will get the benefit, and in only a few minutes the other one will be moored in the harbour, the tide will go out later and it will be left stuck on the mud.

They are harvesting shellfish.