A strong possibility is Children of the Sea (2019) for vibe—but earlier and closer matches could be The Sea Prince and the Fire Child (1981) or Song of the Sea–like stories (though Irish, not anime).
Another closer fit many forget: Haguregumo / obscure OVAs inspired by Ryūtarō Nakamura–era fantasy, or The Princess Arete (2001) for tone (no water magic, but style).
If the water-control scene is central, also look into lesser-known 90s fantasy OVAs like:
Mermaid’s Scar / Rumiko Takahashi OVAs (ocean, sisters, emotional)
The Weathering Continent (1992)
Your description screams pre-2010 arthouse fantasy anime. If none click, I can narrow it further—this is very findable.