[quote=Pathfinder]I would advise against it unless you can make a good way for it to travel with the party quickly.
[/quote]
I was thinking she'd have to rely on others physically, such as having to hold onto someone else to travel fast or over large distance. Also, would a mer-jelly be a type of mermaid or a different race entirely? I know jellies aren't really fish.

Edit: And I just found a better picture than I could have hoped for.
[Hider=Mer-jelly appearance][IMG=http://th04.deviantart.net/fs71/PRE/i/2013/159/7/3/jellyfish_mermaid_and_pearlgirl_by_veboshi-d662ctc.jpg][/hider]