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

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