Fey. Fey creatures can sense magic within 10' of them. Fey creatures are highly magical.
Amorphous. Water spirits are made of water, and as such are immune to critical hits, any attacks or exploits which require targeting a specific hit location, and flanking, and cannot be grabbed or restrained.
Constrict. A medium or smaller creature hit by a water spirit's wave attack is grabbed. The spirit can only grab one target at a time. Grabbed targets can be pulled into the spirit with a second attack, at which point the victim starts drowning and must form a drowning countdown pool.
Immersion. When immersed in water, a water spirit is completely invisible.