This is a real song, referred to by Christopher Pulling in a 1952 book entitled They were Singing and What they Sang About. The opening lines are:
All the girls are lovely by the seaside,
All the girls are lovely by the sea.
It is one of many 19th and early 20th Century music hall songs about girls at the seaside and about the seaside in general. In James Joyce's Ulysses, there is reference to one called Those Lovely Seaside Girls, which was written in 1899; another song, written in 1907, with the title Oh, I Do Like To be Beside the Seaside is still well known today.
Listen on Spotify: Oh, I Do Like To be Beside the Seaside.