Whithout knowing the details of your activity or even which watch you use it's hard to tell, so all that I can advise you is to read these generic tips:
... and if the problem occurred with the Open Water Swim activity, then have a look also at the document Tips to Improve Accuracy of Open Water Swim Activities (it is also linked at the bottom of the document suggested by tmk2, so I am posting it just to make sure you did not miss it)
