Here is some info from the Canadian "Safe Boating Guide". It is intended for recreational boaters but I think this is rather universal;
The Canada Shipping Act
establishes a framework of rules and regulations and incorporates international conventions that shape the behaviour of mariners and boaters alike. One such provision calls for every pleasure craft operator to render assistance, insofar as the operator can do so without serious danger to their own craft or persons on board, to every person on the water who is in danger.
This is geared toward the recreational boater, I'm sure the Coast Guard is somewhat similar but are much better equipt and trained for more adverse/extreme circumstances.
