Question about the second option: Will he/she(?) release the shark girl later and make her following willingly? I mean, the collar is only forcing her to obey; to to enjoy it, right? And how will he/she keep her from drying out?

In other notes:
I suggest you change the formating of the chapters to make it easier to see where the first option ends and where the second begins. That would be especially useful for the reader to skip to the desired part.

[Stuff which happens before the decision]
What do you do?

[Option A] or [option B]

[Option A]
[Content of option A.]

[Option B]
[Content of option B.]

8502103
For your first question, yes, she will follow them(Character B is referred to with gender neutral pronouns, they, them, it, etc.) willingly later, and the camp is close to the lake, so she can go back to the lake when she needs to.

I appreciate your suggestion, I will edit the chapters to fit that in

8502774
Yes, it looks better.
I don't know if it was intentionally or not, but you didn't add the horizontal lines between the options.

This one. You can create it by writing [hr], in case you didn't know.