The first Adult Sea Ponies are somewhat unusual. Not only did Hasbro UK put out two entirely original sea ponies of it's own, they then proceeded to conveniently forget their existence when compiling the My Little Pony Fact File a few years later. The Fact File claimed to contain 'the entire range' of My Little Ponies, but Seaspray, Wavebreaker and Surfdancer were not among the ponies pictured.
It's important first off to note that Seaspray has also been sold with a pink shell, so I suppose some margin for error has to be allowed. However the scan from the insert shows them with their correct names and shells as Hasbro intended :). Surfdancer (blue) is the UK equivalent to Seawinkle, with the same colour shell and body and only a different colour of hair, whilst Seaspray has the same hair colour as Sealight, though they have different shell colours and body colours. Wavebreaker appears to be identical to Wavedancer, but she has a yellow shell instead of a green one (I have a spare of this UK exclusive shell which I will trade for either Sea Breeze's or Wave Dancer's shell ;) Ahem :P)
Surfdancer is the star of one of the UK superstar pony story books when she helps a mermaid, and all three of them appear in the reprint of The Man In The Moon (though the original version includes the AMERICAN sea pony names :-D)