From a human perspective we obviously couldn’t smell them underwater. If one came ashore still in their mermaid form they’d give off a unique indescribable alluring floral scent. It’s apart of their supernatural mystique. It most likely includes various paranormal pheromones since human males are easily beguiled by the bosom baring oceanic beauties.
When they take human form on the night of the full moon the aroma and pheromones are less noticeable but still apparent.