The first step would be to identify the culprit. Most fine lawn species of grass do not tolerate wet conditions. It sounds like there must be a pond grass, or some type of emergent plant that is causing the issue. If the pond sustains aquatic life then certain herbicides should be avoided or used carefully as directed if approved for that particular use. You may be able to hire someone to manually remove the unwanted vegetation, then from there have a seasonal routine to abate the problem. Other options would be to create a landscaped buffer around a portion to help control the issue. Not knowing the full extent of the issue, it is hard to make a recommendation to the best solution