Hey Fetis,
They are at the beach for a party(The players talk about that in scene 1) .
Some information can be found as the story progresses and some information can be found in the description too.
I wanted to add much more details but given the voice acting and the time limitation I wanted the story to gain a faster flow.
Thank You for trying out Attack on the Seaside!! I hope you enjoyed the journey of one shot-ing everyone<3