One Island...This Island is the first, and yet one of the Biggest islands. There are few bits to mention here but here we go:

The First Thing of Note is that the Pokémon Center is quite larger as it contains a Giant Machine that allows you to trade with Ruby, Sapphire & Colosseum. But you need to power it with the Ruby and Sapphire Plates
There are trainers in the waters and hiding in the Grass aswell as Wild Pokémon Such as Fearow
On your path North through the Island are some Hot Springs which allow you to heal your Pokémon after the Many Battles on the way up and also contains a Man who will give you HM06
Also, High Up on this Island in a cave quite far to the north is a Level 50 Moltres. Moved from Victory Road and Now Here. But you can only get to it as soon as you rid the Islands of the Bike Gangs
After Beating the Elite 4, behind where 2 Rockets were, A Cave Opens. This cave hosts Wild Slugma and also has a Braille message but it after that it holds the Ruby Plate

Hover to get area

Here are the Pokémon you will find in the wild on One Island :

In The Grass
	Geodude	Common Common
	Spearow	Common Common
	Fearow	Rare Rare
	Persian	Rare Rare
	Ponyta	Common Common
	Rapidash	Rare Rare
	Tangela	Rare Rare
	Psyduck	Uncommon
	Golduck	Rare
	Slowpoke	Rare
	Slowbro	Rare
	Magmar	Rare
In The Water
	Tentacool	Common Common
	Tentacruel	Rare Rare
	Horsea	Rare Rare
	Gyarados	Rare Rare
In The Cave
	Machop	Common Common
	Machoke	Rare Rare
	Geodude	Common Common
	Graveler	Rare Rare
	Slugma	Common Common

Thats all on Island 1...Click Here for Two Island