Fish does not play like GAT, Dryad doesn't get big very quickly, that leaves it vulnerable for a long time. When i played with dryads, it made me want to overextend to make it fat. All of that investment goes down the tubes when it's removed. That can be a huge tempo loss, there is no reason to risk that.
Cloud of Faeries, Curiousity, Standstill, Daze and Force of Will, which get cast fast and furiously, will all make it bigger, and it is not overextending. I have rarely seen Fish hold back spells anyhow... so many 2nd turn openings have it drop all Clouds and at least one more spell.
There is no validity in saying boas is just as easy to remove, we all know that is untrue. Dryad dies to nearly all targeted removal, it dies to fat in combat, if you cant cast 2 other spells quickly it dies to fire. It dies to most burn spells, remember fork is now unrestricted, there will be much more burn being played. Also as you said it dies to Disk and Keg. In my experience boas is very difficult to remove.
No, I don't think we all know that. First of all, a first turn Boa dies just as easily, and Fish taps out more than any other deck anyhow. Keeping mana open to regenerate, is risky, especially if you rely on having it. A random Ice or Waste effect will leave it just as dead. The ability of the Dryad to grow huge while playing your deck in the background, that sounds more like tempo, anyhow.
Burn marshalls through fish regardless, so I don't think there's much point in worrying about that. I don't think they will bother with killing your creatures anyhow, unless they can fire 2 of them at once, and the fact that the Dryad is a quicker clock is a great boon. Any deck that starts with a turn 1 Chain Lightning to the head doesn't worry about a 2/1 islandwalker anyhow, it will get past it.
The only other targetted spell (other than the burn deck arsenal) that matters is Swords to Plowshares, and that kills both.
Disk and Keg are the only 2 concerns, and sometimes, Stifle and counterspells work wonders. Disk is too slow anyhow, unless you walk into a Drain, and having stifles as Time Walk, Dazes and Spiketails... I think Disk is not a big concern.
Can't win them all anyhow.
Call of the Herd is fat, it's card advantage, and there is no investment involved. You dont need to wait for it to get big, nor start casting spells you should not be to make it deal more damage. It doesn't die to Fire/Ice, and it's incredible in combat.
I have yet to see Fish flashback Call of the Herd. I am not saying it will never happen, I have not seen it happen. The possibility of Card Advantage is not worth much, unless it actually happens. Braingeyser's awesome, if it would actually be used.
On the side, Echoing Truth is widely played, and can really mess up your Call of the Herd idea. That's not a reason to choose anything else, but if you talk possible card advantage, here it is.