There are several different methods of seed dispersal used by plants. One of the most common is wind. Seeds dispersed by wind have small wing attachments, or are so light they are easily carried by the wind and can drift, sometimes for miles, to seek out a location with enough freedom and nutrients to grow. Water dispersal works in a similar way, but the seeds have different characteristics.
Some plants disperse their seeds by rivers so that the river current picks them up and carries them away until they become trapped on a different bank and start to grow. Other plants disperse their seeds in tidal areas or estuaries by which the seeds are carried away into oceans and float for a long time before coming to rest on different sections of coastline.
Seeds that float in water must have especially adapted shells. These shells must be strong, durable and waterproof: If any water leaks inside the seed, the embryo will be ruined. If the seed floats in tidal areas, the shell must also be able to resist corrosion by salt water. These seeds typically have very tough, woody coverings that are difficult to break open.
Seeds that float on water must also be buoyant. Most water-dispersed seeds grow with some type of air cavity inside. Coconuts, for example, are hollow. This air cavity is large enough to counteract the weight of the seed and keep it floating as long as the shell is not pierced.
All seeds have some type of food storage to help the embryo grow when it begins to put down roots. In water-based seeds, the food mixture must survive long periods of time without decaying. This means the food source tends to be tougher and more meaty than that found in more fragile specimens.
The parent tree that produces the seeds must grow near a stream or ocean in order for the seeds to be properly dispersed. This is always the case—barring human intervention or a sudden change in the local geography.