A troublesomely long process if you think about it... what you'll need is the following
-A Pokemon with Dig
-Wailord and Relicanth: Wailmers can be fished while Relicanths are underwater via Dive
-
Basically surf left from Pacifilog and attempt to stay in the middle of the route, as that is the most effective means of getting to the underwater cave where unlocking the Regis take place.

Yeah... I was rather annoyed in this video, sorry for the cursing.