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.

How would you get data back? I'd imagine you'd have to have a surface relay, to a moon-orbiting satellite, then back to earth.

They come to the surface, then get a gps fix (err) and then is picked up by the boat then the data uploaded..... Ok its going to need some minor modifications.

You also need to get from the surface to the water, and from the water back to the surface. Inbetween is a rather huge layer of ice. On Europa it's though to be 10-20 kilometers thick, though there's indications it could be as "thin" as 3km some places