From July to August is offical swim season in Miyazaki.
Beaches in Miyazaki City
Nagisa no Koban Facebook Page with updates if the beach is open
Beaches in Nobeoka
Nobeoka arguably has the best beaches for swimming in Miyazaki Prefecture.
In Nobeoka we recommend Sumie Beach for a nice splash in the ocean.
There is more than enough parking and some shops, but it is very relaxed. It takes about 1.5 hours by highway from Miyazaki City, but it is worth it.
Shimo Aso beach is another good choice if you value
Source: Explore Nobeoka