In a traditional wedding in Ancient Greece, the bride would participate in a ritual bath before the wedding. A sort of purification. Here, Rhea is doing this before her marriage to the Titan king, Cronus.
Themis tries one last time to make Rhea see the danger of what she is doing and end this head long rush towards destruction that she sees the titans on. Rhea, though, sees this as simple jealousy. She was not the chosen one, so she’s going to try and stop the wedding.