The literal meaning for seaweed is an aquatic or ocean plant similar in appearance to long leafy vines. Urban dictionary had some colorful ideas about its meaning, but none specifically linked to urine. I have looked at the game you mentioned and I think it would be possible to explain the reason seaweed and urine are linked without you posting the questions and possible answers as they appeared. The game uses twisted logic and humor so it's very unlikely that any sense can be made of the result without that information.