Sinking can be in sand or water. The act of falling into something constantly usually with difficulty or inability to escape. Drowning is the lack of air specifically in water (or work or something abstract like that)

"Sinking" means something is underwater, and going deeper. It is the opposite of "floating." "Drowning" means that something needs air and cannot get it, because it is underwater. If I toss a rock in a lake, it is sinking, but it is not drowning. (The past tense of "to sink" is "sank.") When the Titanic hit an iceberg, the ship sank, and many of the passengers drowned. Those are the basic meanings. Both words have a large number of related meanings, literal and figurative.