OP I'm very interested to see this thread. I guess I must defer to majority opinion, but I'm very surprised; I've never had a job where the parents told me what I had to do with the kids (short of appointments, lessons, etc.) but rather they'd tell me what the kids might enjoy, or of an activity they heard was going on, and leave it to me to decide how to structure the day. So this is a new issue for me and I would probably feel the same way you do (being in the sun all day, even with a hat, really takes it out of me). I would hope that if you talked to your MB and said basically that, and that you didn't mind staying at the pool all day a couple of times a week but would she be okay if you took the kids to do ______ on the other days, I would HOPE she'd respect you as a person who also has human needs and preferences, and would agree. 7 hours at the pool, especially if it's every day or almost every day, is over the top in my opinion. Even when I take the kids to the beach, which is a big production and a fairly long journey, we don't stay more than three hours!
Listen OP, I would completely understand how you don't want to be at a pool everyday but I would point out to you that this isn't the job for you. With younger kids, you are exactly right. They didn't have prefaces and developmentally were not picking their own activities. The nanny would do this. Older kids do have preferences and the nanny doesn't have the luxury of setting the agenda. The same thing happens with playmates. When the kids are younger, the nanny picks the play dates and its fine if these are with her friends and their charges. When the kids are older they have their own friends and its not appropriate for the nanny to be picking the play dates based on what is more enjoyable to her.
This year we let the nanny go because the kids want to be in sports camps and we have a afternoon very athletic teenager/college student watching them for a few hours. They are outside all day and loving it. There is no way our former nanny could keep up with this. She offered to do it but she would have been miserable. The kids love, love the more active part time nanny. She wouldn't have been the right fit for when they were infants but she's great for their needs now.