My boys went to YMCA daycamp for 5 years in a row. THEY LOVED it.
Here is my answer to your questions:
1. The YMCA is using their already established professional lifeguards to watch your child, PLUS extra folks to be right there with the little ones. They will be just fine. Little ones actually tend to learn to swim BETTER when overprotective moms aren't there. You'll be pleasantly surprised by how much she'll improve. AND you can put her in swimming lessons while she's there too!
2. They aren't outside all day. Talk to the providers. The childcare manager. It wouldn't make sense if there were no place to get away from inclement weather.
3. It is a long day, but it's not ALL running around. They'll watch movies and do sit down crafts too. Either way...she'll sleep GREAT at night. I suggest that you have dinner early.
4. They're young, but they have supervisors. And they're all trained.
My kids did a great job there....and we never had any problems. We loved that there was such a variety of things for them to do. And being able to do swimming lessons while there was SUCH a plus.
Enjoy it. She will. And don't worry!