Once, while this card is in the Graveyard: You can send the top card of your Deck to the Graveyard, then, if it is a WATER monster, Special Summon this card from the Graveyard. You can only use the effect of "Fishborg Planter" once per turn.
Skreech is able to send 2 WATER monsters from your deck to the graveyard when it was destroyed by battle, effectively sending Fishborg Planter and Citadel Whale to the Graveyard. You can Normal Summon another WATER monster, and use Fishborg Planter’s effect to Special Summon it to the Field, and then Special Summon Citadel Whale from your hand or Graveyard.
The "once per turn" part at the end is meant to refer to the physical card, it means after you activated the effect, regardless of whether you milled a WATER or not, you cannot activate the effect of another copy of "Fishborg Planter" in the same turn.