Question

why some seeds float on water

Answer 1

Well, there can be two reasons for seeds floating on water:

(1) The seed may have a density which is less than that of water and thus it floats.

(2) The seed may be having a waxy coating on its surface which helps it to float.

Answer 2

Some seeds float in water due to  a wide number of reasons, they may be lighter in density than the density of water, there might be insects or microbes that have eaten the inner surface of the seeds thus making them hollow, the seeds may have a waxy coating around them that makes them float in the water.

Floating of the seeds in the water helps the seeds to reach long distances and thereby get germination through the water.