it easy , you must use surf then go to route 128
You have to dive beside Sootopolis in different spots until you find one with an entrance, then you will find the sub. dive on route 128 and look for a cavern it should be there
The sea floor cabin.just one square tto the right of route 128 on the pokedex.you'll need rock smash and strength to get around the cave.
its in jagged pass
on route 128, there is a place for you to dive, if you look on the poknav its near the bottom of the darker blue section. there should be a door in the rock go thou and there in the sub, then all you have to do is surface.
route 128
Luvdisc (fishing in Route 128 & Evergrande City) have a 50% chance of holding one and you may also find some in the following locations (Itemfinder may be required):Route 104, Route 105, Route 106, Route 109, Route 115, Route 118, Route 124, Route 126,Route 127, Route 128 and Lilycove City
The underwater cave is located in route 128.
Seafloor cavern in route 128
at route 128
its route 128 in between two islands with a trainer apiece
On Route 128, and in Evergrande City.