Having more success. You really have to control the center squares and use her abilities as much as possible. I think I was being too passive with her. Some tips:
1) Use walls to seal off side flanks to force enemies down the middle, where you lay your traps
2) Use Naiya as a battleforge at the start and then she works great for casting enchantments later
3) Use songs whenever possible
4) Drown works great on Creatures with 8 health or less, don't use it on anything higher
5) Save the initiative for being more aggressive
6) Harpooners and Tidecallers are your bread and butter Creatures
Still no real success with Shoalsdeep Crocodile or Colossal Crab but I think it might just be the fact that they paint a target on them the moment they are summoned.