Southend is the nearest coastal sea town to London and it is very straightforward to get there. You just go to 'West Ham' train station and take the c2c train from there. The ticket machines are in the station and the tickets are about £11 return last time I checked. It takes less than one hour for the train to get there. I think that 'West Ham' train station intersects with the Jubilee line, DLR and a few others so it is very easy to get there. I'm no stranger to Southend, I took my son there for his first day trip to the coast last summer. Southend is on the Thames estuary, the place where the river Thames meets the sea. There is something there for the whole family, it has the longest pleasure pier on earth, beach side cafés, amousement parks, adventure golf, all sorts of things. Plus there is a nice view of the Ocean.
Let's see what happens in 2022.