You can't catch Rayquaza in Undella Town or Bay. You'll need to trade for it or transfer it using the Poke Transfer.
you have to dive into the hole in the water next to undella city
How do you catch rayquaza in Pokemon black and white
Undella Bay
Fish around in Undella bay
There is no legendary Pokemon in Undella Bay.
by action replay
No, you cannot get Rayquaza in Pokémon Black and Pokémon White except for via trading or migrating.
you cant catch rayquaza in Pokemon black
You can only get a rayquaza by transferring it from heartgold or soulsilver or by the GTS.
Between Undella Town and Back City/White Forest
Yes maybe you can in places like black city and undella town after you complete the game. ^^ No. Black City has no wild Pokemon and it is not in White Forest. It is also not available in Undella town. You need to transport it from another generation.
You get Dive in Undella town. The lady that gives it to you is the only women outside.