These Ruling(s) have not been confirmed as applicable to the TCG.[Notes 1]
Since sending 1 WATER monster to the Graveyard is a cost and not an effect, when "Genex Undine" is Summoned and the Turn Player activates the effect, the Turn Player must pay the cost right away, sending the 1 WATER monster from the Deck to the Graveyard. After the effect is activated and all costs paid, the opponent would then be able to chain with an effect like "Effect Veiler".
Sending 1 WATER monster from your Deck to the Graveyard is a cost.
↑Konami Judge Program Rulings are considered unverified, as their source(s) are not publicly viewable. They are not confirmed as applicable to the TCG until they have been officially published by Konami.