The essence of cruiser rules is that an unarmed vessel should not be attacked without warning. It can be fired on only if it repeatedly fails to stop when ordered to do so or resists being boarded by the attacking ship. The armed ship may only intend to search for contraband (such as war materials) when stopping a merchantman. If so, the ship may be allowed on its way, as it must be if it is flying the flag of a non-belligerent, after removal of any contraband. However, if it is intended to take the captured ship as a prize of war, or to destroy it, then adequate steps must be taken to ensure the safety of the crew. This would usually mean taking the crew on board and transporting them to a safe port. It is not usually acceptable to leave the crew in lifeboats. This can only be done if they can be expected to reach safety by themselves and have sufficient supplies and navigational equipment to do so.
Source: Cruiser rules – Wikipedia