You are right that good watch keeping might not prevent a collision otherwise inevitable. On a dark night, you can't see much of anything that isn't well lit. On a dark stormy night you can't see much of anything even if it is lit. But there is far more unlit, unmarked fishing gear (for example) in the ocean than derelict abandon sailboats. Even though I follow these things like a desperate ambulance chasing lawyer, I can't recall having heard or read about a single instance of a collision with a derelict abandon sailboat. Its a big ocean with very few derelicts, dark nights account for maybe 1/4 of the time spent. If you run over one in broad daylight and nice weather, it's on you. When calculating the dangers of an ocean crossing, surely that one gets lost in the round off error.