Nope you surf near the bottom of Union Cave. It will be moving around until you contact it. It comes today, Friday only.
(better explaination) On a Friday go to Union cave. Use surf in the second to last body of water in the cave. When you surf down there will be a ladder, go into the ladder and then you will have to surf again. There is another ladder on the other side, go in it and I think its just another tunnel. So go to the other side and go thru the ladder. This time you will be on this bigger chunk of land. Go down and go surf. You can explore the whole area if you like. While going down there is 4 entries. Go past the first 2 top ones and on the second entry on the left is where you need to go. Go onto the chunk of land and fight the girl up top if you like. Then go down and you will see another girl. Battle her and after your done she will say something about wanting to see this pokemon. Well next to her is another body of water. If you surf in it there will be a Lapras in there. Make sure you have a lot of ultra balls.