Another fun idea, is that rather than sending a submarine, we could send an "upside down rover" to explore the ice that covers the Europan ocean - from below. After all the ice / water interface is a likely place to be of astrobiological interest.
This has been tested in Antarctica, to explore methane seeps from the sea bed. These go up to the ice, and then the convection associated wth the bubbles melts the ice right through to the surface (the bubbles don't change the melting point of the ice, they just help transfer heat from the ocean to the ice).
They call it the Buoyant Rover for Under Ice Expeditions, or BRUIE.