Opus means 'Work' (see 'Magnum Opus', so nothing to do with water.
Walrus is a reference to 'I am the Walrus' by the Beatles.
Squid probably came about because someone thought that it was a funny play on words with quid, i.e. a pound.
Oyster is apparently inspired by the Oysters found in the Thames and the phrase 'The World is your Oyster'.
The others are, as the parent suggests, probably related to things of local significance.