IO starts with the protagonist traveling to Okinawa on a school trip takes a detour after three girls who claims to be her sister. This manga is all in all a romance-mystery with addition of Comedy,Harem and supernatural.

Art:10
If you want to see some outstanding art of underwater/ scenic beauty, I will recommend this as one of the best. The views drawn on first few chapters are enough to lure you to read this entire series. Also the characters are designed pretty well. There were few scenes where you might confuse between Taiyo and Shibuki (they have similar face and shoulder length hair at times), but all in all its great.

Story:7
This is where this series lacks behind. The overall plot is nice.From the start it started building up the suspense, but then there was suddenly a cool down, which kind of spoiled the fun.There were times when it seemed like the story accelerated too much. There were also few places they slowed the time unnecessarily.
The story has a lot of twists (a positive or negative point depending on your choice, honestly at time I got frustrated with so many twists). The last arc started with the huge wham but again it was unable to satisfy my expectation and the series ended with a little disappointment.
All in all, it was a great manga.

Character:9
Great character design, especially the Miyara girls and Taiyo. The true intentions of the researcher, various associations and other side characters were not shown fully till the end which was one of its best point. I found the Mar's charachter a bit unsatisfying and also the fact that there was no other guy in the whole series similar age to Taiyo. but it can be neglected

Overall great manga. I will recommend this one to read it at least once eapacially for the underwater views.