Pay the Toll.
The mishipishu are respected as water spirits by many tribal cultures and secluded communities but they are hardly "guardian spirits". Instead, they are nefarious creatures that take pleasure in playing with their prey-- including humanoids. With humanoids, they demand a toll: if they are hungry, they will demand a tribute of food but, if not, they will demand something precious. If a mishipishu has lived long enough in the same territory it expects a tribute to be placed as soon as humanoids enter its territory and will punish all impudent humanoids that dare to enter their territory. Sometimes communities work out an agreement with a local mishipishu and it will, begrudgingly, guard a waterway in exchange for payment.
They are also known to ally themselves with evil creatures and magic users. They are valuable allies and are considered superior to the inscrutable and alien nature of elementals.
Part Elemental, Part Predator.
Water panthers have many of the same ability as water elementals. They can shift between a solid and liquid form, allowing them to move with an unnatural ease and slide onto offending boats like a rolling tide to attack trespassers. They usually try to capsize these vessels and drown the victims, having no qualm with devouring dead humans, but usually allow at least one member of a party to survive, especially if they seem weak, to spread the word of their presence.
A fully grown mishipishu, or great mishipishu, usually lords over larger territories and often demands large sacrifices. If their demands are not meant, they often dispatch lesser mishipishu, water elementals and other minions to terrorize the locals into paying them respect.
Enemy of Thunderbirds.
Mishipishu loathe thunderbirds and other powerful creatures that share their territory. Their favorite meal is the egg of a thunderbird and a nest full of unguarded thunderbird eggs is often the only thing that can get a mishipishu to travel far from their waters.
