Seeds are defined as: propagules unique to angiosperms and gymnosperms. It is formed by pollination and fertilization of the ovule.
Everyone can think about how to pass the fertilization under water? (Some water lilies are seeded! Maybe is a bulb!)
Therefore, most aquatic plants have no seeds, and most of them are ramets or stems.
To put it bluntly, it is "seaweed mask" and "false syrup seed"!