Slipstream Serpent
Creature — Serpent
This creature can't attack unless defending player controls an Island.
When you control no Islands, sacrifice this creature.
Morph 
(You may cast this card face down as a 2/2 creature for 
. Turn it face up any time for its morph cost.)
Darrell Riche
Rulings
Slipstream Serpent's first ability applies only while you're declaring attackers. If it attacks while face down and then is turned face up, it's still attacking even if the defending player controls no Islands.
(wotc • 2021-03-19)
If a creature attacks a planeswalker, that planeswalker's controller is the defending player.
(wotc • 2021-03-19)
Rulings
Slipstream Serpent's first ability applies only while you're declaring attackers. If it attacks while face down and then is turned face up, it's still attacking even if the defending player controls no Islands.
(wotc • 2021-03-19)
If a creature attacks a planeswalker, that planeswalker's controller is the defending player.
(wotc • 2021-03-19)