What could be the effects of accidentally spilling supercooled water/liquid on an unprotected body surface? Will the body rapidly start losing heat or will the water find a seed and solidify around it ?
 A: The rapidity of heat loss from black body radiation depends only on the temperature of the body and the difference to the temperature of the environment according to the Stefan-Boltzmann law. which describes the power radiated. This will happen in any case for any body immersed (first version of question) in a supercooled liquid if its temperature is higher than the liquid temperature. 
Seed is a small particle , not a human body, where a lot will depend on how the equations turn out. Some parts of the body, hair etc will probably gather frost, but as an extended body has a lot of temperature there will be liquid left in non supercooled form, if the temperature of the body is higher than the supercooled liquid.
The same will be true for pouring supercooled liquid on an extended body. Small protrusions  will probably display frost and bulk normal liquid.
A: Spilling supercooled liquid on your skin would actually be less painful than spilling a similar liquid that wasn't supercooled. This is because when a supercooled liquid freezes it gets warmer due to the latent heat of crystallisation released by the liquid to solid transition.
So suppose we had some supercooled water and some other hypothetical fluid at the same temperature and specific heat and thermal conductivity that wasn't supercooled. When poured on the skin the supercooled water will start freezing and start being warmed by the skin in some messy fashion determined by exactly how the water is poured. You'll end up with an ice water mixture. The other fluid will stay fluid and will just cool the skin without any liquid to solid transition, so it will stay colder than the supercooled water.
