Information About Deep Blue Sea
It appears that 'Deep Blue Sea' was originally not a stand-alone nursery rhyme, but the second verse of one of the many different versions of 'Ring-a-Ring o'Roses'. Just when it became separated into the current stand-alone children's song is unknown.
According to many readers on MumsNet, as well as MamaLisa, the words used in these versions vary between 'Ashes on the water, ashes on the sea'; 'Fishies in the water, fishies in the sea' to the words now used for 'Deep Blue Sea'.
Either way, the fact that 'Deep Blue Sea' forms part of 'Ring-a-Ring o'Roses' means that its origins are somewhat shady. Many believe that this rhyme dates back to the Middle Ages, when Europe was ravaged by plague and describes what happens to those catching the disease. Folklorists do, however reject this idea, partially because:
Symptoms described in the rhyme do not particularly fit those of the Great Plague
The rhyme was not associated with the plague until some time in the mid-20th Century
19th Century and European versions of the rhyme suggest the 'falling down' was not about actual falls, but curtsying or bowing down, both of which were common in many dramatic singing games of the times
There are so many different versions of this nursery rhyme, it is difficult to tell which was the original, making interpretations of its true original meaning almost impossible.
Varying versions of this nursery song were being sung across Europe to the tune still used today during the 1790s. The earliest printed version of 'Ring-a-Ring -'Roses' as it is widely known today, however, did not appear until 1881 (Mother Goose; Kate Greenaway). Learn more about 'Ring-a-Ring o'Roses' at Wikipedia.