Please tell me how to navigate the currents completely. Every site said to navigate the currents, but not how. I spent an hour trying to find the cave, and it didn't turn up.
Ok, since you're still having trouble even with the video, I'll try to answer this with descriptions.
Exit Pacifidlog Town to the west.
Go to the BOTTOM OF THE ROUTE, and enter the currents. If you do not do this, you will miss the narrow channel to the part of the route with the cave. Just stay as close to the bottom as possible, and everything will be fine.
This part is really important. Once you reach a barren (nothing on it) sand island that is shaped like a long rectangle, look for two little sand 'outcroppings' on the west end. Make sure you enter the water between those two outcroppings, or you will miss the dive spot.
Follow the rest of whatever guide you were following.
https://m.youtube.com/watch?v=pQfphSG53Qo -That video is for the trio
https://m.youtube.com/watch?v=3qGMeDe-dYw -This is for Regigigas
Hope I helped!