This page notes what is a Pokemon nest, a Pokemon nest location, and a rare Pokemon nest.
Here are the list of Horsea nests. Some of them are confirmed by players but others are rumor. Attached map screenshots help a player see if players are able to actually capture specific Pokemon in those locations.
To see the details of Horsea, tap the name of the Pokemon above.
What is Go Radar?
Go Radar is a map tool to show the icon of Pokemon which are caught by other players in an area and how long the caught Pokemon stay in there.
Screenshots of Go Radar map is not an enough evidence to see whether an location is a nest or not. The result is surely different by the time when a screenshot was taken. However, it can be a suggestion to help a player to visit an area.
The screenshots were taken around 4pm on August 21.