Well, first we do, but only materials that went on space missions, not regular garbage. For that, the costs would be prohibitive. Its more the stuff of sci fi than real. Again, we do.
Marine debris washups from cruise ships and naval ships of all nations is already a big problem.
Marine dumping of wastes is frequently an international issue. The stuff floats to land and gives us the need to get rid of it all over again. Dumping also damages fish, wildlife and vessels and fishing gear. There is a report on this Report of the Interagency Task Force on Persistent Marine Debris that s probably available from the National Technical Information Service. It is at least 5 years old.