There are 3 forms of nuclear radiation. Alpha, Beta and Gamma. Alpha is the most potent yet, could be stopped by a piece of paper or something over your mouth, so that you do not breath/ingest it in. Beta can be stopped by a piece of wood, or certain clothing and equipment, such as basic hazmat suits. Gamma is X-ray, its the least potent, yet it goes through most things and depending on the fallout, it can be concentrated enough to kill you. Radiation poisoning is just like any other poisoning, think alcahol. In small amounts, you are not going to die, if enough gets into your system, you will be screwed. Most of people that died in hiroshima and nagasaki died many hours after the initial detonation, for that were not aware of how to protect themselves from radiation, whilst they evacuated the area.
Knowing that, it will leak, just look at the soviets dumping their nuclear submarines and waste in the arctic. The severity of such actions have little data to account for, however provided the leak is on the ocean floor, it wouldn't affect the people on the surface. Chances are, it might affect the fishes on the sea bed, but it would unlikely affect us, and if i can recall correctly the united states lost a few nukes over the atlantic.
Radiation is however not safe by any means, but nothing like what you would see in Hollywood. It also have long term genetic concequences, but so do many other agents from serious ones such as agent orange, Mercury or sun light. Oh, and i wouldn't eat those fishes thats near the wreck, or the cows/milk from eastern europe. Radiation can also get into water ways but oceans have such a big volume of water its of very little significance. On the other hand, if it was in a river for a town that gets their drinking water from, thats problematic.