They would need to be a certain weight for the rocket they would ride on, be able to withstand radiation, generate power or install a different power source, be

They would need to be a certain weight for the rocket they would ride on, be able to withstand radiation, generate power or install a different power source, be able to withstand various extreme temperatures, ensure it can "swim" in different oceans (Titan's ocean is not H2O). Not to mention they need to develop a way to deliver the sub to the water safely. I think there are lots of changes here.