if you use temporal on watcher, does it cost accelleration again or is that free?

First off: Temporal Portal can't be used on Watcher at all — it only grants Repeat to the next spell you play, and Thousand-Tailed Watcher is a unit (Rule 820.1.a). So there's no "Repeat" interaction there.

If you mean Temporal Breach (banish a unit, owner plays it back ignoring its cost): Accelerate is not free. "Ignoring its cost" ignores only the base costs (Watcher's 7 energy + 1 Mind power). Accelerate is a separate optional additional cost — you may pay 1 energy + 1 Mind rune to have it enter ready, or skip it and let it enter exhausted (Rules 805.1.a, 204.2). Same rule as playing Watcher off Baited Hook or Z-Drive.