my geese live on grass almost solely, so long as the grass is lush. i offer scratch with a little bit of chicken feed mixed in and they almost completely ignore it. they will nibble at any turkey starter that the turkeys spill, but it seems like it's mostly out of curiosity. if they have access to fresh, healthy grass, they really should be fine. and LOTS of fresh water- they need their buckets cleaned out AT LEAST once a day because they get their water very, very dirty.
as for swimming, mine are pretty terrified of water. they weren't raised around it apparently (they were grown when i got them) and no matter how much i try to teach them to embrace their water-loving side, they won't have anything to do with it. they have a large tub they could get in, but they are content to just stick their head in it to groom. oh, that's another thing: their water buckets need to be deep enough for them to get their whole head in- they clear their nostrils and groom by sticking their head under the water.
being in TX, i'd pay close attention to how hot they get. geese can overheat pretty fast so they need good access to shade and, in case i haven't stressed it enough, lots of water.
tell her good luck!