This is a word play = and a fairly juvenile one at that - my six year old grandson would find this hilarious. To 'wee' is colloquial for to urinate. (Although Americans say 'pee'). So if someone urinates, they would have 'weed'. The joke is that the sea (the ocean) 'weed', (urinated). The full word 'seaweed' of course merely means aquatic plants that grow in the sea. They do say that humour is the hardest thing to translate to another language, and sometimes- well, it just doesn't. I remember being told a number of 'jokes' by my Japanese teacher and found them utterly incomprehensible, even with a full explanation in English!

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.

Need more context. "Peeweed"? Haha I've heard that seaweed makes one's urine smell especially unpleasant, much in the same way that asparagus does.

I've never heard anything like seaweed being related to urination before... seaweed is just, seaweed...

This is a joke for small children which only works in British English, which explains why the American members were as mystified as you were. 'Wee' is a word used to mean urinate. It's like 'pee', but it's gentler and more childish. It's usually the term which British parents teach their children. The past form is 'weed'. So when a child who is a native speaker of GB English hears the phrase 'The seaweed' they understand the equivalent of the Portuguese 'O mar fez xixi'. As you can imagine, this is just the kind of thing which pre-school kiddies would find very funny indeed.