Cats, like dogs and humans, have the ability to get seasick. That being said, animals seem to get over their seasickness more quickly than their owners. In this regard, you may want to gradually get her used to the boat over a series of outings. Although they are not fond of the water, most cats can swim. A piece of carpet or a line of some sort will provide them a way to exit the water in case of an accidental dunk. As for the safety harness, yes, they do exist. They are much like chest harnesses for dogs. That being said, I would recommend getting her a cat pfd. A pfd will give you more time to get her out of the water and often have a loop on the back which you can use a pool to fish her out of the drink.