Sea Flame is fog
A sea flame rises above the ocean, when the seawater is relatively cold (in spring or summer). The warm air coming from the mainland comes above the sea. Due to evaporation of moisture above land, this warm land air can contain quite some moisture. Above the cold sea, the warm air is still cooled down (especially on the open sea). In this way, there can be fog that suddenly comes inland. In a short time the beachgoers see nothing more and the temperature drops dramatically. The sea breeze thus brings the ocean fog inland, and this has been called a naval flame because seamen used to think that the fire was on the coast.
Https://www.youtube.com/watch?v=qzOtL9wu2k0