We went to the blue caves on a Monday in August (Japan Summer holidays). There were easily 100-150 people in the water between the main entrance into the sea and the caves (roughly 200m swim to get to the cave). When we entered the cave there was little to no room to move as the cave was just full of people, almost to the point of being a health hazard.

The swim to the caves was lovely with clear warm water, coral and lots of colourful fishses.

If you plan on snorkeling just bring your own or rent a pair as entering the sea and caves are free. Don't bother with a tourist package/tour as it would be a complete waste of money for what they can offer you.

There are a few beaches in the area within walking distance that are nice to see after you have checked out the caves.

There are facilities by the entrance to the caves which includes: parking, toilets, showers, coin lockers, drink vending machines and a restaurant of sorts. The surrounding township is small but also has a few good options for lunch.

If you are coming from Naha city you can get to the caves by bus. Times are available on Google maps and are fairly accurate, just expect it to be delayed by 5-10min. Travel time was approximately an hour and a half.

Positives: clear water, good snorkeling, nice beaches around the caves you can go to after. Negatives: can be very crowded in summer, cave itself is a bit of a let down.