The way through the Seafoam Island caves is full of twists and turns, and you'll have to puzzle your way through. The complex labyrinth of caves is one of the most intricate that you'll encounter but also hides one of the most powerful allies available, the Legendary Bird Articuno.
Seafoam Islands can't be cleared without a Pokémon that knows Strength. The islands feature a boulders puzzle. Once solved, it permanently remains that way. Setting the boulders in the right place will halt the strong currents, allowing surfing the water.
After you enter the cave, go east and push the boulder into the hole to the north. Now go to the steps north of the entrance and go up. Grab the Ice Heal. Within this cave you'll get your first chance to find a Seel, and more rarely, its evolution Dewgong.
Articuno
The legendary Pokémon Articuno resides in the depths of the Seafoam Island caves. The two boulders on the first floor can be used to block the current on the third basement level, which makes it easier to get around, but is not necessary for catching Articuno. It's the two boulders on the third basement level that are important: knock the other boulders out of the way so you can get to the ones in line with the holes (if you mess up, leave this floor and they'll reset), then jump in after them, swim to the island in the northwest, and you'll find Articuno.
Save the game before facing Articuno. You're going to want to capture it, as it's your only one and the only chance. Come adequately prepared, with Ultra balls aplenty, since legendary Pokémon are the most difficult Pokémon to obtain in terms of catch rate.
#144 Articuno
The Ice bird, Articuno, is the one of the three Legendary Birds native to Kanto. At an impressive level 50, Articuno sports the powerful Ice Beam, and its Mist move will nullify status moves while active, making it more difficult to catch.
Once caught, in order to make best use of Articuno during your playthrough, you may want to bolster its moveset with TMs/HMs. It can learn one Water-type move, Water Pulse, to take care of Rock-types, whose moves the Ice/Flying-type bird has a 4x weakness to. You could also teach it Fly or Aerial Ace to take advantage of its Flying-type STAB.
Seel is a decent Water-type Pokémon, with average stats all around, and a learnset consisting of only Ice and Normal-type moves. This would be quite annoying, as Seel will not benefit from Ice-type STAB advantages until it evolves into Dewgong, an Ice/Water Pokémon, at level 34. Fortunately, since you need Surf to reach the Seafoam Islands in the first place, you can teach it a fantastic STAB move right away.
Seel can certainly fill a spot in your team, Water and Ice being two almost essential types for your future fights against the Elite Four. Sadly, there are simply more powerful and versatile Water/Ice Pokémon you can choose instead, such as Lapras or Cloyster. As such, Seel is likely best left in your PC unless you're a fan of the evolution line.
