I was on an Alaska cruise with Regent Seven Seas in July during sailaway from Ketchikan (I think) sitting outside by the smoking area chatting with one of the ship's officers. A group of older (60's-70's) men came outside, told everyone out there what they were going to do and that they were all welcome to enjoy with them.
I continued chatting with the officer - who was there for another 10 minutes or so - while the group smoked their joint and headed back inside. Neither of us reacted at all, beyond declining the offer to join in.
Other than that, the only mention I saw of marijuana was in ship's documentation reminding us that we weren't supposed to be bringing it or using it on board.