Water Shrine was first intended to be a Reaction and I dropped the reaction^^. I wanted something linked to "the next turn" and attacks but without being a reaction card.
Yes, "swap" is better (in fact in French the verb "échanger" means both "exchange" "trade" and "swap" and the word "swap" is closer to the French verb "permuter". )
Ok, which one do you prefer ?
-Swap any number of cards from your hand with the same number of cards from your mat
-Put any number of cards from your hand on your Shrine mat and that many cards from your Shrine mat into your hand.
-You may put any number of cards from your hand on your Shrine mat. Take an equal number of cards from your Shrine mat into your hand.
The 3rd one is GendoIkari's proposal, it seems a bit long to me so I dropped the "you may" (because it includes 0) and factorized the hand->mat and mat->hand clauses with the verb put to have the 2nd one.
On a side note : I forgot but cards on the mat are returned to deck as the game ends.