Pollination by water
Flowers pollinated by water are called hydrophilous flowers. In hydrophilous flowers, pistillate flowers and staminate flowers are in most cases situated separately.
When mature, the staminate flowers detach from the plant and float on the water. When such a flower drifts to the stigma of a pistillate flower, pollination occurs.
Example of a flower pollinated by water — Vallisneria.
Figure to be added
The whole flower travels
This is the one agent where it is not the pollen that makes the journey but the entire staminate flower. It is set loose like a small boat and carried on the surface. The pistillate flower, meanwhile, is held right at the surface on a long stalk, exactly where the drifting flowers will reach it.
Water plant does not mean water-pollinated
Most aquatic plants — the lotus and the water lily among them — hold their flowers above the water and are pollinated by insects like any land plant. Hydrophily is rare, and Vallisneria is the example to remember.