Plants have adapted many different ways to spread seeds around their ecosystem in order to reproduce and out compete other plants that grow within the same conditions. for plants that live in or next to bodies of water, floating is a typical method for seed dispersal. Some seeds have the ability to float for hundreds or thousands of miles in search of new places to grow.
The coconut palm, Cocos nucifera, is possibly the best known plant that has a floating seed. Floating through the ocean is the primary way coconut palms have spread throughout the isolated islands of the tropics and colonized almost every beach in the world that doesn't receive a frost. The large nuts have an on board air pocket in the center cavity that makes them very buoyant and they have their own supply of fresh water in the form of coconut milk to keep them hydrated under the pressures of the intense tropical sun and salt water. The thick outer husk protects a coconut from pounding waves and rocky seashores. A coconut can float for up to one year before it finally rests on a suitable space, germinates and starts to grow a new tree.
Mangroves are the single most important trees for the health and sustainability of tropical and subtropical intertidal zones where freshwater drains into a saltwater estuary or salt marsh on the edge of the ocean. Very few trees are able to tolerate the changing salinity of the water as the tides come in and out. Red mangroves, Rhizophora mangle, are key to preventing erosion from hurricanes and create new land masses on sandbars and coral reefs as the forest grows outwards. The trees form almost impenetrable jungles right in the water and along the shore providing a vital nursery habitat for many small fish, fry, and invertebrates that feed a coral reef.
The seed of the red mangrove germinates on the tree after pollination and grows a modified root from the bottom of the seed that is called a propagule. When ripe, this falls off the tree into the water and can float in full oceanic salt water for up to a year before it lands and takes root and starts to grow a new tree. This has allowed mangroves to spread around the world, usually in conjunction with coral reefs.
The blue lotus has been a staple in western mythology since before writing existed. Ancient Egyptians prized the flowers as a symbol of the creation of the universe and for it's mildly psychoactive properties. These flowers are said to be what the intoxicated Lotus Eaters were consuming in Greek mythology.
The blue lotus, Nymphaea nouchali var. caerulea, is native to the Nile river in Africa but has been spread around the world. Like many other species of Nymphaea, after pollination the flower sinks to the bottom and grows a pear shaped fruit full of thousands of small seeds. When ripe, the seeds are released and all are individually enclosed within a thin membrane that holds air pockets allowing them to float up and away from the parent plant. After a few days the membrane disintegrates and the seeds sink to the substrate in another part of the river or pond where they germinate and grow a new lotus plant.