Isekai de Tochi o Katte Noujou o Tsukurou - Vol. 1 Ch. 4 - Has a crisis finally reached our relaxing lifestyle enthusiast!

[VividlyLucid29]

It makes no sense to call him master

 

[Snailman32]

Why would a mermaid make alchemical weapons? It doesn't seem like the sort of thing that would effective underwater