Non-swimming enemies will enter water if player is water walking
In vanilla Morrowind land-based enemies like rats, nix hounds and kagouti will stop short if the player enters water, although they are not explicitly de-aggroed and can be observed to still attempt to face the player. This occurs whether the player is submerged in water or standing on it.
In OpenMW, they are de-aggroed immediately if the player is submerged in water but maintain pursuit if they are water-walking - even though they are now moving in water themselves and have no swimming animation.