As you have heard from many of the Mom's here, it is recommended not to use sunscreen (and bug repelants) until your baby is 6 months. My Pedi. said that this is because they can develop a high sensitivity to it later on. I know it is not easy, especially where you will be outside on vacation, but I would go with the umbrella shade and SPF clothing. Another site that sells them is www.onestepahead.com.
My daughter has sensitive skin and I found that California Baby (Target, Whole Foods) works great for her.
Give a quick call to your pediatrician to get their opinion and enjoy your time at the lake!