Several possible leads over at the VWML/Roud Folksong Index perhaps?

http://library.efdss.org/cgi-bin/query.cgi?cross=on&index_roud=on&index_roudbroadside=on&query=seagull&start=2&output=Record&access=off

Ginette Dunn's Fellowship of Song (1980) pp.241-242 has a song 'Seagull' referenced in it - I don't have the book and Googlebooks only shows parts of it. It's on singing traditions of East Suffolk.

Roud 10834 'Is the Story True' has an opening line 'Seagull, gentle seagull'. It appears in Cheney, Mormon Songs from the Rocky Mountains (1968) pp.78-79

The Opies reference, Roud 20742, is probably of the fairly well-known 'Seagull, seagull sit on the shore' variety.