We tend to think of water as being the most gentle of the elements. The adjectives used to describe it symbolically all tend to center around things like “nurturing” and “healing” and “comforting”. The truth is, though, that the largest single body of water on the planet comes not in the form of warm hotsprings or gentle babbling brooks. It’s ocean, and ocean is wild and ferocious and drowns people. It is also the source of life, something that is no conundrum to anyone who understands the nature of nature itself. In the ancient North, life was harsh and every part of nature was clearly seen as having both positive and negative aspects. The destructive essence of everything was not euphemized, but honored with clear-eyed reverence.
This can be seen in the nature of the Sea-Gods. They are not (or not merely) the nurturing, welcoming waters; they are the sea that eats people. As in all things, when you confront the elements through the Northern Tradition, you start with the natural disasters. The Sea-Gods can bear you up and teach you all about the powers of water, or they can drown you.
Other European ancestors understood this as well. The sea gods—from Aegir to Poseidon to Tiamat—may have been propitiated, perhaps even desperately, but their powers were rarely invoked. The sea was seen as too dangerous an entity to attempt to harness or control. Today, when global warming is raising sea level and creating more tumultuous weather, we may again find ourselves in the situation of regarding the sea with its original ancient mix of fear and respect, which it has never really ceased to merit.
Aegir, Ran, and the Nine Undines – as well as other, smaller, ocean spirits – are understood as having a capricious nature. They may be charming one moment and drag you under bloodthirstily the next. The Nine Sisters (of which “ordinary” mermaids are pale copies) are not what humans would consider beautiful; they can put on a glamour to appear beautiful to unsuspecting sailors, but they have teeth and claws and all the power - and danger - of the ocean itself. Deal with them carefully. I have found that so long as you are respectful to them and to their domain and gift them properly, they will respond well. They have strong senses for sniffing out hypocrisy, however, and if you are thinking to merely use their power and give nothing back, they will know, and you will not be safe.