Link finds the Iron Booots in the Ice Cavern, where they are guarded by a White Wolfos. Link needs them to gain access to the Water Temple under Lake Hylia. The boots make him sink down to bottom of any body of water deep enough to swim in. Link also needs the Iron Boots in the Shadow Temple because of all the powerful fans that he must resist.
The Iron Boots look identical to the Kokiri Boots in every way except that they have a strip of iron running along the soles.
The Iron Boots are found in Ice Ring Island. Link needs them to get out of that island, but he needs them mostly because he needs to get inside the Wind Temple. Like in the last game, they are used to resist and even push against the force of the wind. Link also needs them to push down certain switches, and pull down statues by using them in conjunction with the Hookshot.
Link can not use them to sink to the bottom of a body of water, as he automatically takes them off as soon as entering the water.
Mayor Bo gives Link the Iron Boots after Link defeats him in a sumo wrestling match. When Link uses them, he can stop rolling Gorons (which is needed to get up Death Mountain Trail), and he must use them to beat the Goron Elder, Gor Coron in a sumo match. Link can also use them to walk on magnetic walls and roofs. Until the Zora Armor is aquired, the Iron Boots are the only way to sink to the bottom of a body of water.
The Iron Boots are vital for defeating some bosses, and are required for getting through some of the puzzles in the game. Interestingly, if Link stands on quicksand while equiped these boots, he immediately sinks.
As an improvement over Ocarina of Time, the boots are treated as an item rather than equipment, and can be removed or worn on demand, without the need to pause the game.
The Iron Boots are an item that are confirmed to appear in The Legend of Zelda: Skyward Sword.