After the second World War Omega introduced their Seamaster models, based on their experience of supplying army forces with robust waterproof watches. In 1948 the first mentioning of Seamaster was visible on the dial of Omega watches. 

Omega Seamaster De Ville Automatic Date Crosshair (1963) - Calibre 562

The Seamaster De Ville series

Before the De Ville became a stand-alone collection of watches, Omega used this De Ville name for some of the dress watches of the Seamaster collection. The De Ville was born in 1960 as Omega's elegant dress watch and produced until 1967. It was self-winding and water-resistant, and had a monocoque case which means the movement had to be removed from the front of the case.  

Seahorse or Hippocampus logo

The seahorse logo of Omega, or hippocampus complete with with bridles, was designed by Jean-Pierre Borle - engraver at Omega - in the 1950's. Aparently he was inspired during a trip to Venice when he saw the hippocampi on the gondolas.

In 1958, the hippocampus became the symbol of the Omega Seamaster as a means to signify the watch's water-resitance. First etched into the caseback, later embossed.

Shortly after its introduction, Omega also used it's seahorse insignia for other tool watch models such as Railmaster and Speedmaster. That is why today the seahorse logo is still present on the Speedmaster model