So in the VN, the island culture has a phrase: "dosukoi" which is like a big insult. "Dosukoi" is also a sumo wrestling chant. So the joke here is that he thought she was inviting him for sex cause he's dumb.
This is short, I'll do a rough TL if someone wants to type set it.
Page 4: "Gotta do CPR"
Page 5: "....I can't hold back"
Page 6: "..So...So soft"
Page 7: "Nnfuun", "CUMMING" "hnn?"
Page 8: "Wh...What have you done?"
Page 9: "Welll, you were drowning so I did CPR and..." "...m" "Huh?" "YOU SCUM!" "hm"
Page 10: "Wha" "Why??? Are you crazy!!?" "No!" " Stop!" "Nha!" "Don't! Stop this!" "AAhn"
Page 11: "Why!" "Ah" "Are you...Doing this" "Ahn!" "Hnn!" "Eh??? But..." "You told me to 'Come!' didn't you" "Augh" "N-No!" "Ah" "Ah" "I said you're 'scum'!"
Page 12: "(As expected, this island's girls are so assertive)" "Okay, I'm cumming inside" "N-N-N-NOT INSIDE!"
Page 13: "NOOOOOOOO!"
Page 14: "That sort of evening event at the pool.." "It would definitely happen♡"