Considering the spectrum of abilities the Tidesages have, along with their general depiction, seems to cover all three somewhat evenly (at least in my opinion) I’d say it mostly just boils down to what you prefer mechanically and aesthetically. Obviously it would be easier to obtain an authentic transmog with a cloth wearer, however Shaman arguably has the most visuals appropriate for the role at their disposal.
In summary, at least in regards to class choice, I don’t think it matters that much since they don’t entirely fall into any single one we currently have; the closest two perhaps being Shaman and Mage, in terms of capabilities.
I currently don’t have a Tidesage myself. I had a concept for one that I was going to use a Warrior for but the idea was meant to be somewhat atypical.
On the off chance you were looking for tips my server’s forums has a guide someone wrote…