If you're looking for anime similar to Umi Kara no Shisha, you might like these titles. All recommendations are made by Anime-Planet users like you!
Four months ago aliens invaded Earth, but they're not obliterating cities or abducting huge amounts of people. Instead, they send one single alien to the surface each day and, for reasons unknown, teenager Kakashi is the only person in the world who can fight them. However, this duty takes a huge toll on the boy's psyche, and after a while his memories have all but disappeared. Only his broken cell phone is there to remind him of the life he left behind, but Kakashi is afraid to fix it - for what if there are no new messages, and nobody cares that he's no longer around?
The plot, characters, story, etc. of these two short films have little to nothing in common. However, they were both made (created, directed, animated, etc.) entirely by a single person, which is pretty rad.
So if you enjoyed seeing what a single person with a tiny budget could acheive in one of these shows, you may want to check out the other- it's a neat compare/contrast.