most druids I see are wolf builds, not many casters.

and the casters I do see vary kinda widely in destiny choice, so here is my question.

what destiny for a pure caster druid (water ele form)

neither shiradi nor primal avatar compared to other caster destinies are pure caster destinies, and while there are goodies in heading toward the arcane or divine destinies you end up missing out on the caster level bonus from shiradi or primal.

I'm currently leaning towards shiradi *IF* the shiradi procs on every tick of spells like earthquake, ice storm, etc.

but as always have come to ask my betters for advice.

PS: draconic or magister for pure wizard PM?

I've got no current endgame type Druid in Shiradi experience.

However, I did play a couple pure Druid lives in Shiradi and just about everything triggers Shiradi effects, even Fog Cloud. 

Some (and I can't recall which) only hit once a mob enters and may need to be kited.

Stack a bunch of AOE effects for profit.  Ice Storm, Storm of Vengeance, Earthquake, Fire Seeds.  Dot as needed.