latest video lesson from SciShow Kids is all about how submarines work. The video does a nice job of covering the basics of how submarines are sunk and how they are returned to the surface. A little demonstration that you could create in your classroom is included in the video.
My only criticism of the video is that I wish there was a bit more explanation about how oxygen is supplied in a submarine. Perhaps the answer to that question could be the start of a lesson in your classroom.