If you sale all seven of them you might come across an isle or two, but the one with the treasure is in just a single what?
Options:
- sea
- river
- ocean
- body of water
- auditorium
Let's think now! Step-by-step reasoning:

Show your work and conclude with the exact phrasing "Therefore, the final answer is [answer]." where [answer] is just the final number or expression that solves the problem.
user
Isle is an island or peninsula, especially a small one. If you sail all seven of them you might come across an isle or two, but the one with the treasure is in just a single sea.
So, the final answer is sea.