It's only available on Fridays. When you first enter Union Cave, go to the last water area (on the bottom). The rest is pretty simple, just surf and walk. Until you get to a large area, with water and a bit of land in between.
A Lapras should be swimming in the last water area there. Just surf up to it and try interacting with it, and you will go into a battle with it, it's Lv 20.
As darkgoku said you can find Lapras in Union cave but ONLY ONE per Friday . This is the same with HG/SS .See here for informations .
You can find it in the union cave which is to the north of route 33 and to the south of route32