Not a botanist, but their tadpoles swim. Plants without this feature will relocate the sperm themselves.

e: As a result of learning this, I also learned that molluscs, arthropods, and vertebrates all evolved brains independently, and it's actually normal for molluscs to have distributed brains rather than it being a neat quirk of a few examples. Arthropod brains are also more or less distributed depending on their specific needs.