Spotted Seal is the summation of Selkie's many Spotted Seal-related abilities. He can do anything a regular seal can do, even on land.
Spotted Seal grants its user enhanced strength and agility on land. It also grants them increased swimming speed as well as the ability to use echolocation underwater.
As the famous Sea Rescue Hero, Selkie is known for using his Quirk on water-related missions. He uses his incredible swimming ability and echolocation to search wide areas for threats. His enhanced strength can be used to overwhelm villains in a fight. 
- My Hero Academia Anime: Episode 32.