Kelp juice is mentioned when Karen reveals the Kelp Shake's ingredients to be 99% kelp juice.
"Chum Bucket Supreme"
Kelp juice makes its physical debut in this episode. When Patrick's head is set on fire from mass confusion, Plankton urges him to use the kelp juice that he's drinking to put it out. Patrick just ignores this warning and offers him a sip of the drink. Fed up with his stupidity, Plankton snatches the cup out of Patrick's hand and throws its contents over Patrick's head to put out the fire.