To be honest, I've been down there countless of times and I still have the feeling there are some things I haven't seen yet, but I can definitely show you some of my favourite spots.

[Carbuncle's comments have him chuckle.]

Well, hopefully, you are not too set on seeing Magikarp. Apart from the lost individual here and there, they don't generally hang around the reef. There are tons of other water pokemon though. Even some pretty rare ones.