Submersibles : Why they matter to underwater colonization

Submersibles and submarines have been around for centuries, so what can we learn from them? How can we use them to further the goal of colonizing the ocean? https://en.wikipedia.org/wiki/Personal_submarine https://en.wikipedia.org/wiki/History_of_submarines https://www.uboatworx.com/ https://tritonsubs.com/ http://www.deepflight.com/ https://www.seamagine.com/small-submarines-models.html https://en.wikipedia.org/wiki/Supercavitation