This is a great video from The Verge which explains how SpaceX achieved it's ground, err, water breaking** accomplishment of landing a rocket at sea.
Water landings are important because while rocket-based ships take off from land-based platforms, the fact that the Earth is mostly covered in water means that by the time the boosters are ready to come home, they're over water-- not land. Landing by heading straight down is far more fuel (and thus cost) efficient than travelling back to the landing pads they launched from.
This is the sort of tech that could allow us to get a real version of the Utopia Planetia "dry docks" up and running for preparation to a mission to Mars, IO, Titan, and who knows where else.
**That doesn't sound right either...