Oh wow. They made a manga of this.
.......Hmm. How do I put this...it seems like it's going to be a somewhat lacking adaptation. The web novel I'm reading at the moment is quite good, but here they blitz through a lot of the fine details that made things interesting.
Well, in summary:
1. Makoto here is very, very weak stat-wise. If a normal villager is level 10, then a normal otherworlder starts at level 80 and our MC starts at level 0.
2. He has so little mana that he can only cast one spell before exhausting it too much to cast anything again. The reason he was able to do that big move to clear out the goblins is because he spent a full year grinding up his Water Magic proficiency so that he can manipulate natural water at basically zero cost.
3. He's basically a water bender, which is a skill I think the average person can think of a lot of badass ways to use, but most people don't have proficiency levels as high as Makoto and it's one of the lower rated combat magic elements in general, so this world doesn't realize how useful it is if applied correctly.