While most women are insane, some of them are not.
That fact that you are asking means you already know she is the kind that will be upset.
Take her with you, for her. Go back for you.
She doesn't want to see the fish. She want's to be included. She want's to see the fish, with you.
Or she just want's a free trip. And she will suck every last drop of enjoyment from your day.
Whatever.
If you don't take her she will be sitting around talking with her friends asking if this qualifies you as a jerk. They always, ALWAYS say "Yep!" If she had any doubt before, she will be convinced by the time you get back. And you will pay.
When you are done paying, and you forget about this, she will remind you. And you will pay.
Women: They do not want what they say they want. It is a test.
If the "other" is a guy, just distract him with something shiny. You'll be back before he knows you left.
And you can't go by yourself first and take her later. Because it won't be "a special moment of discovery shared in the relationship" it will be "old and used and 2nd rate 2nd hand" and you will be a "jerk" and she really will suck out all the fun.
And you will pay.