Seaweeds are those who 'go with the flow' and 'sway with the wave'. They're clueless people who go along with what a crowd believes/says/does, like seaweed swaying with the waves of the sea.
Ken: Hm, I think I'll go buy a new shaver.
Barbie: Oh my gosh! Me too!
Ken: I think I'll go buy the new packs of jumbo condoms. My friend says they're high quality.
Barbie: I totally know what he's talking about! I was thinking of buying them too!
Ken: I have a problem.. my girl agrees with everything I say...
Friend: What a seaweed..