While it is possible to take the train to Kyotango, it looks like the closest stations are Amino and Kitsuonsen, and you would still need a bus/taxi to get to the beach. Therefore, I think this is trip best left to the automobilically (wat) mobile. Luckily, renting a car in Japan is almost as easy as ordering pizza online (though both can be exceedingly frustrating at times).
Most swimming areas are named and will have parking lots, beach-side food, toilets, showers, and even camping sites, for those into that. There are some beach-side hotels as well, but those are quite expensive. Up side, they usually include an equally expensive but delicious dinner or breakfast.