Water primrose is one of the easy ones. Most anything will be effective on it. 2-4-D may be the cheapest. If you dont have very much and cant find 2-4-D in small quantities, Ortho brush killer(tricopyr) or Eliminator brand weed killer(glyphosate) both at Walmart will do a good job. Put some liquid detergent in the mixture to make it stick and spread out on the leaves better. I know some will say it must be labeled for pond use, which increases the cost. Your choice.