Are tokens permanents?
You are correct.  According to the comprehensive rules (emphasis mine):


  110.1. A permanent is a card or token on the battlefield. A permanent remains on the battlefield indefinitely. A card or token becomes a permanent as it enters the battlefield and it stops being a permanent as it’s moved to another zone by an effect or rule


and


  110.5. Some effects put tokens onto the battlefield. A token is a marker used to represent any permanent that isn’t represented by a card.


So tokens on the battlefield are permanents, and if they are not lands Devastation Tide will return them to their owner's hand (and since they're tokens, they will immediately cease to exist after leaving the battlefield).