The plot is identical. Almost too similar, to the point that Umi no Hajimari looks like a total copy from My Girl. Both main characters are young males that were dumped by their girlfriends and years later find out that the ex-girlfriend had passed away. In the funeral both of them find out that the ex-girlfriend had a secret child, a young girl, and that they were actually the father. The details on the stories diverge, but overall they look identical to me. ^^"
Both have male leads who are single daddies. Both leads are from Sutato agency, the black rangers of their respective groups, Snow Man’s Meguro Ren (member color: black) and SixTONES’ Matsumura Hokuto (member color: black). Plot wise, well, very different. With Umi no, you’d probably cry every episode. With Sionji-san, it’s a total laugh trip. I’m still watching episode 2 but it’s really funny. It’s a good balance.