This game is quite complex and can be hard to master. There really aren't many videos out which cover how to play Submarines, but there are many videos regarding Destroyer play. You can learn some positioning concepts from Destroyers that will translate to the stealthy, underwater play of Submarines~
First and foremost, don't be afraid to have space between you and the enemy. As you play more, you'll identify what openings exist and how to make use of them!