Don't make her wear it in the pool or while running around if you can help it - it'll affect her depth perception and make her more clumsy.
My older two daughters have to wear eye patches for a couple of hours every day. They HATE it! And I can't blame them, but it's for their own good so I have to make them.
I do use a reward system. We went to the dollar store and I had them each pick out a smallish (it holds about 2 cups) jar with a LID. Then they decorated them. Every day that they wear their patches without a fit, they get to put a marble in their jar. Once the jar is too full to close, we go to the store and they get to choose a prize. Since it takes so long to fill the jar, we've only really bought 3 prizes so far and it's been about a year. Not too bad.
When my girls try to throw a tantrum about their patches, I remind them that they're allowed to not like it but they're NOT allowed to yell and throw a fit because their upset; that's not how we behave in our house. They usually calm down and do their time. If they insist on having a fit, I treat it like any other tantrum.
My oldest started wearing the patch when she was 2. So I know what a struggle it is.