@Kiani
...dam water pulls u down...
Dam do not have the upthrust which running water such as rivres have. Irregularities, gravel and stones throughs the running water back up, like wise any body in it. Canals are specifically designed with irregular bed.
In the sea, salt in the water makes a body bouy. A man can float on the surface of The well known Dead sea (called becuase of virtually no sea life) because of the high level of salts in it.
Now comming to the question. ...dam water pulls u down...
In the dam,
1. These is no upthrust of River
2. The water is fresh i.e. no salt
3. The weight of still water, which super imposes the drowning tendency.