It doesn't.
The only way you can teach it this is if your playing Black,White,Ruby,Sapphire,Emerald
and you teach it HM08/HM06
Dewgong learns Dive at level 45.
It's actually Barboach and it evolves at level 30 into Whiscash
Wishcash does not evolve! it evolves from barboach
barboach? http://pokemon.marriland.com/platinum/pokedex/barboach/ look at the bottom
it needs level 18 if it did not learn it at level 12 when it was a shinx
Dewgong learns Dive at level 45.
Barboach evolves into Whiscash at level 30 :]
Barboach evolves into Whiscash at level 30.
It can't learn it whatsoever
you can learn HMs at any level, but only if you have a compatible pokemon.
Barboach evolves INTO Whiscash at Level 30
Barboach evolves INTO Whiscash at Level 30
Barboach evolves INTO Whiscash at Level 30
barboach evolves into whiscash at level 30
Barboach > lvl 30 > Whiscash
Dive is not an HM/TM anymore so it can't learn dive, but you can get it from a different region so if you trade a Pokemon holding that move then take it off them it can learn dive.
I dont think it does because I have a level 100 wishcash (no cheat) and it never learnd it. I got wishcash as a level 9 barboach.