mirage island, is found completly random, you need to have pokemon in your party that has specific digits/values that can open mirage island... this is completely random, not all pokemon can do this, to check whether you can access mirage island talk to the old man in pacifilog (wat ever) town that always looks out the window towards the right, if he says he can see mirage island, than just surf towards that direction and look for it =D if not... bad luck, keep trying
'that wierd fish' evolves into a milotic, and is called Feebas, it's found on 6 random tiles of water along route 109 (or was it 119?) basicali, the water route that runs all the way from the weather station down towards berry master's house. Feebas can be caught in those 6 water tiles, which is generated completely randomly, but withint those 6 tiles you catch nothing but feebas, i suggest you fish each tile one by one until you find one.. and catch a load of them to make up for lost time spent searching for it... becoz when ever you mix records or change something in the game (get on TV again for example) those tiles change again... so be sure to catch heaps before you leave that tile (there are 6, but you'll be lucky to find one)
Frank