Where can I get a Pokemon that can learn dive?

Almost every Water Pokemon can dive. And Water Pokemon are the most common in the Hoen region(Ruby,Sapphire), and almost everyone knows how to catch a Water Pokemon, mostly by Fishing or using Surf on water you will eventually encounter a Water Pokemon, here are examples of common Water Pokemon:

Wingull: almost everywhere but mostly by Surfing.

Tentacool: by Surfing and fishing with Old Rod.

Wailmer: Fishing with Super Rod.

Of course there are alot more than those note that the above can be found everywhere -and you have to teach the Pokemon "Dive"- if you don't know where it is then listen up:

When your in front of the Poke Center door in Mossdeep city take the following steps:
3 steps down
13 steps right
5 steps down
6 steps right
4 steps down
10 steps right
7 steps up(you should see a white rock in front of you)
6 steps left
6 steps up
1 step left
9 steps up
3 steps right
1 step up

You should see a bald man, talk to him and he will give you Dive! In the house next to him there is a Fisherman who will give you a Super Rod!

All water-type pokemon can learn HM Dive.

Pelipper can't learn dive
But he can learn surf !