The Zoological Society of London (ZSL) is appealing for volunteers to join the search for dolphins, porpoises and seals in the Thames Estuary.
Ecosystem change is a major threat to marine mammals
The survey will help direct future conservation plans for these and other animals that use the famous waterway.
At the moment, it is not known how many of these creatures visit the Thames - or in which areas they turn up most.
The census will help join the dots, and the ZSL believes it will help protect these species in Britain's waters.
Grey seals are among the rarest in the world: the UK population of approximately 124,000 represents about 40% of the global population.
It is estimated that there are just 300 bottlenose dolphins in UK waters and 350,000 harbour porpoises.
Dolphins and seals are often spotted in the Thames by members of the public, who are an untapped resource in terms of data collection, the ZSL believes.
Grey seals are among the rarest in the world
"We are looking to involve the general public as well as many other river users in this survey," said Alison Shaw, ZSL marine and freshwater conservation programme manager.
Recent sightings in the River Thames and Thames Estuary have included bottlenose dolphins, harbour porpoises, common seals and grey seals.
Although these creatures are protected, the ZSL believes conservation plans are only really effective if you have a clear picture of what you are dealing with.
They hope that the data collected from public sightings will help them decide which areas of the Thames should become protected zones.
The main threats to marine mammals in the UK are thought to be ecosystem change, fisheries by-catch, chemical contamination and boat traffic.
If the public survey can help researchers gain a proper understanding of the numbers and movements of seals, dolphins and porpoises, they might be able to better predict how human activity will affect them in the future.
"Assessing the consequences of development activities requires long term monitoring of these animals," Dr Shaw said.
"The data ZSL collects on marine mammal movements may prove invaluable in assessing the impact of new operations in the Thames Estuary."
If you would like to take part, visit the www.zsl.org website.