When {Floodtide Serpent} ETB's , do I return the enchantment when he enters or any time he wants to attack?
It states on the card (this cost is paid as attackers are declared.) so if you want to attack with him, you pay the cost.
So if I want to bounce back my {Boon Satyr}, I have to attack with the serpent?
If you want the serpent to bounce the satyre, then yes.
It has to attack yes