I don't think you're entirely wrong here. Water Cannon has a low potency even just compared to base spells of other jobs at that level. Fire, for example, is 180 potency. Stone has a potency of 140. Add to that blue mage weapons have no weapon damage and no stats, and you have an issue where killing mobs in the field at your level takes a lot of casts of that spell. So I do think that blue mage spell potencies might need a buff (though I'm still only at level 35, so I'm holding final judgment until I get higher level and fully geared out)

As far as dungeons, I do share the concern. Right now finding parties to go through them to learn spells isn't too bad since so many people are wanting to do the same, but imagine someone picking up the job as a new player in a year from now. If you can't use DF to try and learn all these abilities, you're going to have trouble trying to get the spells you need, especially when so many of them come from dungeons and trials.