They can be powerful terraformers (Growth, CreateWater, DrainWater and WaterPath) or healers (Growth, Heal, GreaterHeal and FullHeal), but they have a few other potential skills for other situations as well. Also let's not forget all the possible skills they can receive from their parents.
However do note that they cannot equip weapons or armor, getting two misc slots and two consumable slots. This makes a Naiad's skillset very important.
Naiads can save you a lot on upkeep costs. As they have both Photosynthesis and Aquatic, they make excellent fishers at Home, acquiring food without eating any. Selling the rare fish they catch can also cover their salaries, essentially making them free to keep.
You must have a Naiad in your party to get the recruitment event for Nyva in deep water tiles.