Undella Bay in those dark spots in the water (not ripples) transports you to Abssyal Ruins or whatevar
Undella Bay
In the darker patches of waters while Surfing.
Undella Town, and you use dive in four dark water spots to get to the abyssal ruins.
any water Pokemon can use dive
In Undella City, there is a girl off to the left of the Pokemon center in a white dress. Talk to her, and she will give you the HM Dive. You can then teach it to a Pokemon and enter the Abyssal Ruins.
the dive hm. the lday in undella town gives it to you.
There is no HM for Dive in SoulSilver, but you can get it by transferring a Pokemon who knows dive into SoulSilver the Pal Park. but you can get dive as a HM in Pokemon black and white but not in soulsilver,heartgold,platinum,diamond,and pearl
Since dive is not a HM in firered you will have to trade a Pokemon to a friend that has ruby, sapphire or emerald then using that game use the HM Dive to teach the move Dive to that Pokemon trade it back so firered can have that Pokemon again it will still have Dive however you cannot use it to dive underwater.
Dive is absent in Pokemon pearl,diamond,platinum,heartgold,and soulsilver . But will return in Black/White =-)
The only thing you can dive for is the abyssal ruins.
You get the abilty to use the move Dive from Tate and Liza in Mossdeep and then you can teach your Pokemon to Dive! X3 POKEMON-1001
you can not, you can get a Pokemon that knows dive in battle but you cant use it outside of battle.