If I play Thousand-Tailed Watcher with Baited Hook, do I need to pay the accelerate cost or is it free?

Yes, you can choose to pay the Accelerate cost if you wish, but it is not required.

According to RiftJudge FAQ #10495 and the associated rulings (#5335, #7120), when you play a unit using Baited Hook, you are playing that unit while ignoring its base energy and power costs. However, Accelerate is an optional additional cost, not a base cost.

How it works:

1. Resolve Baited Hook: You kill a friendly unit, look at the top 5 cards, and choose Thousand-Tailed Watcher to play.
2. Playing the Unit: Because you are "playing" the unit, you go through the standard process of playing a card (Rule 349).
3. Cost Determination: Baited Hook sets the base cost of Thousand-Tailed Watcher to zero.
4. Optional Costs: You are then presented with the opportunity to pay any optional additional costs the card has. Since Accelerate is an optional additional cost (Rule 805.2), you may choose to pay it.
   • If you pay the Accelerate cost (1 energy and 1 mind power), the unit enters the battlefield readied.
   • If you choose not to pay the Accelerate cost, the unit enters the battlefield exhausted (as it is normally played).

Regardless of whether you choose to accelerate it or not, the unit is considered "played," meaning its "When you play me" ability will trigger normally.