You can choose no targets if you don't want to double the counters on any permanents.

To double the number of each kind of counter on a permanent, put another counter on it for each counter it already has. Effects that interact with counters being put onto permanents (such as that of Corpsejack Menace or Fathom Mage) apply if appropriate. For example, if Corpsejack Menace has two +1/+1 counters and a divinity counter on it, Deepglow Skate's ability gives Corpsejack Menace four more +1/+1 counters and one more divinity counter.

As Deepglow Skate's ability resolves, you must double each kind of counter on the permanents it targets.