Both Umi no Misaki and IO are about the male lead who visits an island (Okinawa in the case of IO), where he then meets a girl of his interest. This then goes on further to bring in multiple female leads that are attracted to the main charatcer.
Both mangas have a focus of love and mysteries that are what really make the story a enjoyable read.
Also the art feels very similar and is enjoyable to look at as well as to read.
If you enjoy the romance setting and mystery to the story, you'll likely enjoy reading IO.
Sawako Kuronuma is just like any other high school girl who wants to make friends and be useful. The only problem is she bears a worrying resemblance to Sadako from 'The Ring!' Because of her reputation, people are not only terrified of her, but small dogs even bark in fear at her presence; in fact, the only person in school who will talk to her is the lively class hottie, Kazehaya. As the pair spends more time together, Kazehaya slowly begins to bring Sawako out of her shell and soon their feelings for each other develop further. Though with her crippling insecurities, lack of social skills, and a series of cruel rumors and misunderstandings, it seems that Sawako's dream of a normal life won’t be quite so easy to obtain.
Both of these series feature girls who have lived isolated lives until their teens, and now suddenly find themselves in male-female relationtionsips for the first time in their lives. The girls in both series have a great deal of uncertainty regarding what they're supposed to do, understanding their own feelings, understanding their partner's feelings, communicating the thoughts & desires effectively, etc.
Umi no Misaki is another manga that starts with a city boy arriving on a remote island and getting involved in a mystery revolving around local religion and a number of women. The biggest difference is, that in one the mystery gets abandoned in favor of borderline H (that would be Kyuusen no Shima), while in the other in favor of a (almost) typical harem (and this would be Umi no Misaki). If you found Kyuusen no Shima to have an interesting setting, than you could give Umi no Misaki a try. I can't really say I can recommend Kyuusen no Shima to fans of Umi no Misaki, but due to the similarities mentioned above, I'll still leave this as a two-way recommendation.
Note: While Umi no Misaki for most part doesn't go beyond light fanservice, that doesn't hold true in the latter volumes where mature content can be found.
Both Umi no Misaki and Maga Tsuki feature a male lead who wants to properly respond to the feelings of the girls in his harem. That in itself is quite unusual and could make those series good (not too mention a good match), but...
Maga Tsuki is still a fairly generic harem series despite of that. Not to mention that, the male lead there gets the short end of stick most of the time. As for Umi no Misaki, the guy there gets treated really well for a change, but there is just way too much of the girls drooling and lusting over him (no, really). Oh, and did I mention it get's mature later on?