Piwraithe the Ghost Pirate

If a face-up WATER monster(s) you control, except "Piwraithe the Ghost Pirate", is destroyed by battle or card effect, while this card is in your GY: You can Special Summon this card (but banish it when it leaves the field), and if you do, it gains 100 ATK for each WATER monster in your GY, until the end of the next turn. You can only use this effect of "Piwraithe the Ghost Pirate" once per turn.