Location: Mibaru Beach is about a 30-40 minute drive from Naha. I don't know about public transportation because I didn't use it, but if you have a car, it's worth the drive because the water is very clear and there are unique rock formations, making the beach beautiful. Parking is paid (500 yen).
Experience: You can take a transparent-bottom boat (about 12-seater) out to sea and observe underwater fish. The experience is short, about 20 minutes, but you can observe a variety of fish closely and clearly at four different points. It feels a bit different than seeing them in an aquarium. I think it's a good alternative if you can't dive. Pictures and names of frequently seen fish are posted on the boat. The captain kindly explains at the observation points, but only in Japanese.
Safety: Personal safety equipment such as life jackets are not provided separately, but the boat has a roof and you can only ride sitting down, so it is not very dangerous.