When in doubt read the help files:
"help seaweed"
Seaweed, while technically a 'pill' in some forms, grants the ability to
breathe water and is not considered to be magical in nature. Therans do
not find this property any more unusual than a toxic mushroom poisoning
its consumer.

Doing a quick search netted me this as well, which shows that has been in the help files from atleast 2005.
http://forums.carrionfields.com/dc/dcboard.php?az=show_mesg&forum=6&topic_id=8504&mesg_id=8505&page=