Pollination by water (hydrophily):-

• It is quite rare. It is limited to about 30 genera, mostly monocotyledons. E.g. Vallisneria & Hydrilla (fresh water), Zostera (marine seagrasses), etc.
• As against this, water is a regular mode of transport for the male gametes among the lower plants. It is believed, particularly for some bryophytes & pteridophytes, that their distribution is limited because of the need for water
  for the transport of male gametes and fertilisation.
• In Vallisneria, the female flower reaches the surface of water by the long stalk and the male flowers or pollen grains are released on to the surface of water. They are carried by water currents and reach the female flowers.
• In sea grasses, female flowers remain submerged in water. Pollen grains are long and ribbon-like. They are carried inside the water and reach the stigma.
• The pollen grains of most of the water-pollinated species have a mucilaginous covering to protect them from wetting.
• Not all aquatic plants use hydrophily. In most of the aquatic plants (water hyacinth, water lily, etc), the flowers emerge above the level of water for entomophily or anemophily.
• Wind and water pollinated flowers are not very colourful and do not produce nectar.