I’m going to confess I use this easy approach -
I just put it up as one of the taxa discussed above - brown/red/green algae etc.- depending on what I think it looks like (I put in a little more effort than just the colour, though!) Sometimes I’m wrong, and then an identifier for the entered taxon moves it out of that taxon (and into the right one, or into a broad category).
I now tend to have some idea about what I’m entering my seaweed observations as. But earlier, when I was clueless, I used the exact same approach, and it did get me and my observations on the right track.
As far as I know, there is really no parent taxon for ‘seaweeds’ that is safe to enter and simultaneously get noticed by identifiers. This is true for certain other taxa as well (including animals), and I see this method as the only way to get it going somewhere. Since data with coarse-level IDs will hardly ever be relied upon for serious use, I think this is a low-risk route.