So, with all the recent discoveries and almost certainly true theories that a few of Jupiter and Saturn's moons could have a subsurface ocean, notably Enceladus and Titan, would an ~10 meter long submarine carried into space and 'landing' on the surface (Titan) or drilling its' way through the ice (Enceladus / Ganymede) actually be effective at sending environment-related information back to Earth ?
There are plenty of plans out there to explore the 'oceans' of outer moons such as Europa or Titan with submarine drones. Woods Hole Institude has long been a leader in developing such scientific equipment, so you can get an idea at what the first submarine drones might be like by perusing their website.
The plans would be different for the different planets, and they will require some refining after more probes are sent. In general, Titan's oceans are of methane and open to the surface. They would simply require a drone to touch down on them. Enceladus's oceans may be more accessible due to the cryovolcanism going on. We'd have to know more about that moon before we could make plans on how to explore it.
Europa, Ganymede, an Callisto all probably have a thick water ice crust over a liquid water ocean. In my 'drill through the ice' link, I show that conditions for those drilling expeditions are actually not that dissimilar from those on the Russian drilling into Lake Vostok, as far as temperatures and pressures go, so there is a demonstrated technology to expand from. After drilling the appropriate hole (which could take years), an appropriately sized submarine drone could be send down to explore.