To be sinking in a dream suggests a loss of confidence. We may be unhappy at something we have done, and feel hampered by the circumstances around us. We may feel we are losing ground within a relationship or situation.
To see someone else sinking would suggest we are aware of a difficulty that perhaps needs our help. What we are sinking into could be important.
To be sinking in water would suggest a particular emotion is threatening to engulf us.
To be sinking in quicksand or a bog indicates that we feel there is no safe ground for us. Becoming conscious of a sink or basin suggests that some kind of cleansing is necessary.