One more issue is that it's prohibitively expensive and very challenging technically to explore the bottom of oceans and seas properly. Any exploration needs a

One more issue is that it's prohibitively expensive and very challenging technically to explore the bottom of oceans and seas properly. Any exploration needs a lot of support from the surface, equipment that can stand a very taxing environment (perhaps more so than in space), and the water covered parts of the planet are huge. Unless it's for oil very few bother surveying under those bodies of water.