There are two primary reasons for shielding your new home in space, to keep radiation out and to keep you air in. While I'm more than willing to admit that the second reason is very important; the first one will kill you just as dead, only it will take longer and be far more painful. Depending on the region of space you occupy, you can expect to be bombarded by such wonderful phenomenon as UVs, gamma rays, X rays, alpha particles, stray neutrons, ultra-relativistic particles, antimatter, micrometeorites, space junk, ejecta from asteroid mining, and all sorts of other fun things that hiding at the bottom of a gravity well behind a nice thick atmosphere would keep you safe from. While most of the radiation hazards would pose no significant threat so long as you're only exposed to them for a short while, they will play utter havoc with on-board electronics, and prolonged exposure can spell death in as little as a couple days in highly active areas of space. And then there's the actual physical bodies, hurtling through space with enough force to punch a hole clean through your hull, just dreaming of when they get a chance to absolutely ruin your day.
Hollowed out asteroid - with enough rock and ice between you and space it will take a seriously sized rock to damage structural integrity, and about the only particles that will get through a dozen meters of rock and ice are neutrinos, and they don't really interact with anything anyway.
Piling ice on the outside of your hull - sometimes shielding fails at the most inopportune time, or you're entering a part of space your current shielding is just not designed to handle. A few meters of ice on the outside of your hull will stop basically anything that can be stopped.
Magnetic Shielding - while passive shielding needs to deal with the effects of secondary radiation let off when the primary radiation strikes it, magnetic shielding is an active method that creates an artificial magnetosphere around the ship to deflect and divert incoming radiation. However it provides absolutely no protection from physical threats.
Metal Laminate - Using a combination of sheets of metal and highly radio-absorptive polymers combined with a generally convex hull shape, this type of passive shielding is significantly lighter than pure water based shielding while providing similar levels of protection from both radiation and physical threats. Also, so long as the ship isn't re-entering atmospheres on a regular basis, the constant exposure to small particles in space with polish the hull into a nice sheen.
Ablative Armor - An extremely light armor made primarily to protect against high velocity impacts from small objects, the armor is designed to allow the object through causing it to crack into numerous smaller pieces and tumble around inside the armor layers without ever damaging the hull. Often seen on older civilian freighters, which explains the pitted appearance of their outer hulls.
Active Defenses - Small railguns meant to target larger space objects and either shatter them or divert them from a collision course, generally unreliable at high travel velocities and poor at tracking small objects.
Deflector Shields - Exactly what it say