We should mind the origin of the race and not stretch too much. They're from the tent. And while they love the surface, the sea is only a concept they picked on recently and humans would outclass them there (except in piracy maybe). So that would limit watery reactions. There is already themes to explore, like madness, art or romantic imagery turned deadly (giant flowers throwing spines at enemies?)
Sirens will be an option if there is bird people to corrupt, same with fish people (or nagas?). It should be noted that DF does not support aquatic creatures as citizens, nor fliers, so it might be underwhelming.
While we're talking about the theme, there is hints of Lovecraftian mythology too with rat things and Yog Shothoth, but I'd like to step away from this as there is already enough content (see Bellweasel, they were knee deep into what's pretty much Innsmouth goodness). If the warlocks want the two mentioned creatures, we can transfer these pets to them.
On the other hand, I put the work in progress scripts aside to hunt some bugs. The metallurgist's furnace has been restored, some labors renamed and adjusted to MDF's distribution. Corrupting prisoners will use the negociation's job (formerly soap making in vanilla). A new building will receive both chemist and alchemist reactions, it will allow the making of bloodsteel and ironbone and later on more fun stuff with warpstone.
It is highly possible that the fireball liquid is the cause of the burning caravans. It may have been produced by some obscure let's-bring-creature-extract mechanism. I'll try to reproduce this on a longer world gen and see if adding a SPECIAL tag helps.