Talos,
You will need four Monkey figurines to open an elevator that can bring you to the serpent wand. This wand is neccesary to complete Serpent Temple.
As for Serpent Idol - it is not truly needed but you can find it by swimming (yes, not too easy) the underwater path in the room near back entrance to the Temple. And do not forget to push the lever!
Good luck, hope this will help!
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Sincerely Yours,
Tanda