I think you should be able to walk on dry land, only for a short period of time or your speed is realy slow.
For example: Say you were doing a quest or something and there are lot's of different pools you have to search, and there's little land in between them. You're gonna want to use aquatic form if you have the option. But as soon as you're done with one pool, and you need to take a quick hop over land, you're just gonna change back. Where's the conveniance in that?