This is the sort of mystery that has always appealed to me, the Scooby Doo franchise has often done the sea and ocean set episodes very well. The same goes to episodes with sea creatures for villains. Was really impressed by "The Siren's Song" on first watch, especially the atmosphere, the character of the mermaid, Velma's role and the very hyped up ending. Almost everything about it was excellent and had almost no problem with it until very much later on.
On recent rewatch, and it has been rewatched quite a few times, it is still an excellent episode and one of the best of the season. And for pretty much the same reasons as on first watch, with the good things making an even stronger impression, yet with more to appreciate. So actually "The Siren's Song" is one of the better on rewatch kind of 'Scooby Doo: Mystery Incorporated' episodes, which this reviewer has found to be true of many of the great and more episodes of the show.
It is hard to find a starting point in reviewing "The Siren's Song", but the animation as ever is terrific. Especially the eerie fog effects and the character animation of the frightening but alluring mermaid, the very meaning of a siren. The music has the right amount of liveliness and atmosphere, the mermaid's music really does entice which is a big part of the allure and the creepiness. The writing is beautifully balanced, with a great balance of humour (the all you can gorge and the hilarious references to Scrappy and Flim Flam, loved Fred's reason for recognising the latter and how there was acknowledgement of Scrappy being a hated character) and intriguing drama (everything with Velma and Amy and the ending).
Furthermore, the story is enormously compelling, with a fun, creepy mystery (the spine shivering setting helps hugely) that isn't too dark or take itself too seriously while not being too tongue in cheek at the same time. Even more intriguing is the Velma and Amy subplot and the ending where the truth of a major character is revealed, and in a way where one feels for Velma and her conflicted feelings. The ending is pretty much set up, which is true of a vast majority of the show's endings. The Fish freaks do give the creeps.
Not to mention that the gang are all spot on and Velma's appearance here is one of her most interesting ones. The voice acting is excellent.
My only issue, and it has always been the case with all my watches of "The Siren's Song", is the premature over obviousness of the responsible's identity with the suspects being too few.
Concluding, absolutely great. 9/10.