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The Seaside Cave only exists in the Pokémon Black 2 and White 2 games, not in Pokémon X and Y. It basically connects Undella Bay to Route 21.
In Pokémon Black 2 and Pokémon White 2, the Wild Pokémon that can be found within the Seaside Cave would include Psyduck, Golduck, Seel, Dewgong, Woobat, Frillish, Boldore, Tynamo and Jellicent. Frillish and Seel can be found while Surfing. Seel, Jellicent and Dewgong can be found in the surfacing fish while Surfing. Additionally Excadrill can be found in the Dust clouds.
Mistralton cave
Mistralton cave
Axew is available in both black and white, it is found in Mistralton Cave.
you get it in wellsping cave
In Pokémon White 2, in order to get the Crustle in the Seaside Cave that is blocking your way to the Plasma Frigate to move out of the way is to find Colress in Route 22. Once you are at Route 22, you will see both Colress and Terrakion however you will not be required to battle Terrakion at the present time. Once you interact with Colress, he will give you his Colress MCHN which will allow you to move the Crustle that is blocking your way in the Seaside Cave.
Celestial tower off of Mistralton City, after they escape from the cave
Route 18
If you are talking about Pokemon Black Version 2, then go to Mistrolen Cave
It'll be on the ground on Floor 1 in Chargestone Cave.
1. Global Trade 2.Get the Nationaldex and There might be a Pokemon Outbreak in a Cave. Hope helped :3