from personal experience of blacking out/fainting aboard when I was on holiday once, I had a really bad ear infection which my doctor said was fine, and ok to go on holiday...but it was on a 'ferry' to Spain in choppy seas, and you know what that does to you inner ear= huge infection spread, complete inside of ears swollen so ear canal size of pin hole. Pain like a sledge hammer. Went to A&E in Spain where they gave me huge dose of pain killer to buttock which gave me nausea so they pushed a big bin in front of me which was filled with dirty rotten bandages covered in blood due to all of the above===faint lol
I woke about a minute later when I came round I kind of knew what has happened because I could feel myself falling just before it happened but I couldn't move after i was out cold, you don't faint and instantly move.