Avoid putting sunblock on your forehead. When you sweat, it may run into your eyes. Cover your forehead instead with a wide brimmed hat (Gerry Lopez style).
Also after applying sunbock, wash your hands off. Otherwise they can be slippery and can be an issue in holding your paddle.
Safety note: Laird Hamilton and one other I know suggested not wearing sunblock at all due to damaging the environment and that the chemicals in sunblock may give you cancer. For me, a guy from Irish/Scottish decent and blue eyes, I burn easily and not wearing it may give me cancer. Choose what works for you. I use Sol Sunguard, a Seattle based company with products which last for quite a while after being immersed in water.