Water spots on fiberglass

What is the best way to get stubborn water spots off of fiberglass boats? I've tried using Babes Boat Bright and Lucas slick mist but there are still some spots on the boat and the cowling that I can't seem to get off. Thanks guys

strong vinegar and water, spray on and let sit a minute then just wipe.

+1
On really tough spots I will use straight vinegar.

Mr Clean Magic Eraser cleaned my cowling very well. Make sure to wet the marker/sponge first. Had water spots that were practically baked into the cowling and they came right off.