King of the Seven Seas: When Arthur Curry occupies water terrain, he can use Invulnerability. When Arthur Curry does not occupy water terrain, he can use Toughness.

Aquatic Rejuvenation: When Arthur Curry occupies water terrain, he can use Close Combat Expert and Regeneration.

What happens if Arthur Curry occupies water terrain, but then my opponent TKs an object under Arthur. Is he now occupying water terrain or hindering terrain?
He occupies both. He is still in water, so his special powers will work just fine, but since he is also in hindering terrain he can modify his defense against ranged attacks, he will halve his speed value, and all the other effects associated with hindering terrain.