Marine Cave is a cave found underwater after defeating the Elite Four. It is on Routes 105, 125, 127 or 129. You need to go to the weather center and he'll tell you where it is. However if you take too long finding it, it will move on
It's a basic cave bit first, then the Misty cave
In the Misty Cave, you will see Kyogre. Go to him and you battle him. Kyogre is at Level 70 so be prepared
You can run and it'll still be there after, but defeating it or catching it rectifies the problem, & Thats all that is in Marine Cave

Marine Cave Picture Locations

Route 105
Route 125
Route 127
Route 129