Pokémon Mystery Dungeon: Blue Rescue Team and Red Rescue Team/Northwind Field

After you've beaten Entei and Raikou, you'll have access to here. It's a 30 floor dungeon with plenty of Water types(though there are a couple of Ice types along with a couple of other Pokemon). Like the other two fields before it, there's a lot of traps, so bring in a Pokemon with the Trap Seer IQ skill to make things easier. Once you've traversed the frigid landscape and defeated Suicune, he'll hear you out and add the final part to the Clear Wing. This turns it into the Rainbow Wing, thus granting access to Mt. Faraway to challenge the Rainbow Pokemon, Ho-Oh.

Items
Items include water stones which are common on the last two floors. The Lunar Ribbon(used to evolve Eevee into Umbreon if it's IQ is high enough) can be found locked up on floor 20. Various Gummis and Orbs can also be found here inside the walls.

It's recommended that you bring several Reviver Seeds as well as Oran Berries(or Sitrus Berries if you have enough) since Politoed and Delibird will cause you serious problems if you're not careful.

Enemies
A minor jump in difficulty here compared to the last field. The main things to worry about are Politoed's Perish Song move and Delibird's Present. Perish Song affects every Pokémon on the floor, and inflicts a condition that KO's in a few turns while Present's damage ignores defense and does set damage(40, 80, 120, or healing you by 80HP). Heal seeds and reviver seeds will be your friends because of this. Beware of Muk, Delibird, and Delcatty since they're attacks are really annoying(Delcatty has Attract, which is devastating in Monster Houses and a pain in general). Try to recruit a Poliwag, since they are level 50 and can evolve all the way to Politoed or Poliwrath, plus they have the Water Absorb ability to heal if they get hit with a Water type move.

Boss Battle
Suicune Level 45

Moves: Bite, Aurora Beam, Gust, and Rain Dance

Since Suicune doesn't have any Water type moves at the level you face it at(which is odd since it should have BubbleBeam which it learns at level 11 in Gen III), so don't be relying on Water Absorb for battling it. If you come in with a Grass type, watch out for Gust and Aurora Beam. Electric types are at an advantage here thanks to being strong against it as well as being able to take advantage of Rain Dance if they have Thunder.

Also, Suicune can't be recruited on your first visit, so you'll have to head up through the dungeon and face Suicune again(probably a few times since it's recruitment rate is so low) if you want it on your team.