Hi B.
I have 4 children and I live in a beach community, so we are always in the sun, and honestly when my children were babies we would go to the beach and I would lather them up with Waterbabies SPF 50. Thing is is just to limit the amount of time the baby is exposed to the sun, maybe also cover her with a light blanket or an umbrella of some sort rather than covering every inch of her in heavy clothes and just give her water to drink her bottle (if she is's a bottle baby).
It's completely your decision as to whether or not to use sunscreen, but I will tell you I have been using it on my kids since practically the day they were born, and my pediatrician has never once told me not to, if the choice is lather her up or chance her getting skin cancer, I will lather up any time cuz you can't live your life inside until she's 6 months old. Just use your judgment, and watch to make sure she's not allergic to the lotion as babies skin is very sensitive.
Good luck.