source- https://www.dlsite.com/maniax/work/=/product_id/RJ01589593.html

The rest of Waiwai Seisak stories here https://www.dlsite.com/maniax/circle/profile/=/maker_id/RG41427.html

Premise - "The place I went to with my senior martial artist student was a nudist beach..."
The dojo master has passed away... His last wish was— "Scatter my ashes... in that sea where we have so many memories..." The two disciples fly there by plane— and the sea where their master had so many memories... turns out to be a nudist beach where everyone gets naked!!
The two martial artists are unwillingly left completely naked... but the younger disciple is flustered by the sight of his older sister naked—!! Then they face the beach's rule: "Erection is forbidden on this beach!!"

What will the two of them do---!!