Umisho is a very ecchi manga about the hi-jinks of a mostly-girls swimming club, as seen through the eyes of their (male) manager, who can't swim.
The "can't swim" part is the source of most of what little plot there is: childhood trauma related to some mysterious memory of a mermaid, a mysterious beautiful transfer student up who seems to know him from the past joins the team... skip about 100 chapters of swim team ecchi comedy, and you can guess what happens.
The art is solid. The swimming scenes look good (at least to someone who never swam competitively before), the girls are all cute
The humor is standard ecchi fare, but done well. The asexual adonis team captain is hilarious, as is the big-breasted and shy girl who is a closet pervert. The main character is a standard milquetoast harem lead; the main girl is cute and hyperactive, but mostly exists to light a match under whatever explosive situation is otherwise developing.
The romance is cute and sweet, but if that's what you are here for, there really isn't much of it. A couple chapters at the beginning, a few scattered throughout the middle, and a very rushed (if happy) ending. There isn't even much harem here; the girls mostly interact with each other, with the guy passively looking on.
Still, if you like ecchi shounen comedies, you can do an awful lot worse than Umisho. Don't expect deep plot or much romance--this isn't even To Love Ru or Nisekoi, much less Love Hina. Instead, expect lots of cute girls in swimsuits doing crazy things to each other, often hilariously.