Good evening Océanne,
Here is an interpretation that will surely help you.
Water is indeed the origin of life, since our body is composed of 70% water and, in the embryonic stage, we spend nine months in a liquid... It is there that we unconsciously experience our first emotions, living through our mother's body.
Water therefore represents our emotions in dream language, but also our capacity to transform and evolve, since throughout our lives we have this contact with water, waters in the broad sense of the term.
By extension, the sea symbolizes all the vastness and multitude of emotions that make us up, that surround us… They can be calm, even troubled, or on the contrary, unleashed or limpid. It is the aspect of the sea that describes our emotional state at the moment. One can also push the reflection further by looking at what is in this sea, such as small fish or even enormous monsters. These are as many indications that show us what we are.
The sea also holds a whole potential for transformation and evolution, since the sea is in perpetual motion; it is an element that gives life but can also take it… a creative or destructive power, once again, it all depends on the state of the dreamer.
Starting from that premise, I would say at first glance that your mother may be afraid of letting her emotions run wild (surely for fear of “drowning” in them), but that once she takes that step, she will realize that everything is clear. She has within her the power needed to initiate changes, and that even at her deepest core, she is calm and serene… just like that sea in which she swims “like a fish in water”! Perhaps she needs to make a decision at this point that would force radical changes in her life, or that would slowly lead her to evolve into a different state… In any case, this dream is very positive, whatever her actual situation may be.
There you go, I hope I've been a bit of help, sorry for taking so long…
Amandine.