No, it's not a good manga.
It's a hentai that doesn't want to admit it's one, or that's been watered down in order to barely pass censorship.
It is Valkyrie Drive, the series being very overt with the fanservice is a given. If Breakout! and Bhikkhunism aren't as overt as Mermaid is because they were published in Famitsu. The Mermaid manga, being published in a magazine focused on more mature audiences could be as explicit as the anime itself.
It's just intended as fap material for male teens. The story is almost irrelevant.
If you don't like it that's fine, but that doesn't mean the story isn't good. In fact, it gives a good amount of characterization to both the main couple and the villain of the setting.
I know of 3. Siren (this one) being a mobage + continuation manga, Bhikkhuni being beat-em-up game + manga, And Mermaid being the anime.All of them sharing the same premise of girls being incarcerated on an island for being infected by a virus that turns them into weapons (and does other things) when they get excited. They also share the yuri-ish ecchi tone and general character design guidelines.
Something I really liked was the fact that the three series (with its respective supplementary material) elaborated different aspects about the setting, so the more you read/watched/played the richer the entire premise became.
Though, those I know something about, Siren and Mermaid have non-yuri distractions often enough to bother me. Siren's player character being explicitly male (though if manga is anything to go by, he doesn't get romantically involved with main girls) invokes old "only guy in a strictly female environment" trope I dislike. Mermaid had Mamori's crush on Governer (I know), tentacle + faceless sand guys "fanservice", and the whole ending debacle with non-relationship main girls ended up in.
Siren having a male self-insert as the protagonist was a huge misstep that badly hurt the game and likely, influenced its early demise. They tried to right the course once the fanbase expressed its disappointment (like the manga dropping him entirely and upping the yuri tenfold and the game reducing his role and making sure Setsuna and Urara would be a couple in the final event of the game) but the damage was already done.
As for Mermaid, Mirei and Mamori did end up becoming a formal couple, that was the entire reason they could achieve the Valkyrie Drive, it was just not stated in words but their actions made that abundantly clear.
