*If you're interested in reading this manga, I advise against reading the whole review, therefore only read the TL;DR at the bottom.*
You have the summary here on MAL, so no point in wasting time explaining that.
The MC's aquaphobia is used as an topic for conversation with every girl in the manga, whom he later has sexual intercourse. The basic premise is that one of the girls he meets knows who he is, but the mystery is obscured by the many hentai-like ecchi scenes (specifically, one per chapter).
The story is basically that: MC meets girl, talks to her about his condition, has sex with
her (mind you he has a girlfriend) and then it's like nothing ever happened and he goes back to his girl.
*His fear of water is treated horribly. In the first chapter he passes out because a little bit of water splashed on his face, but a few pages later he's fine with entering the pool. Really?
Main reason to read "Hanma Meido" is the ecchi. And its done very well.
All girls are sexy, their bodies are wonderful, even if their facial expressions aren't phenomenal and even, at times, quite displeasing. Clothing is also very well drawn and erotic.
Other details, like the background, are very poor and forgettable.
They are as shallow as the pools in this little manga. They don't have ANY depth; They barely have a single characteristic to define them. The protagonist simply lusts after every girl he sets his eyes on, and these, who are a deformed cookie-cut trope cliché, simply let him do as he pleases.
Even though this manga is so bad it's insulting, if you like ecchi you will enjoy it.
It's as simple as that.
"Hanma Meido" is so horribly written it hurts, so just skip through the pages and you will get the gist of the story. Then, appreciate the ecchi that you "came" to see.