March is at the end of the rainy season, but Bali has been zapping the clouds with lasers lately in order to hold off the rain to finish a high priority toll road. Not a great idea for the environment, but the good news is that there hasn't been much rain this year so you should be fine.

The places that may concern you with the runoff would be Kuta Beach and Seminyak. I usually recommend people head to Sanur, which is much nicer in my opinion.

Hope that helps.

Find more info about the best beaches in Bali at: http://vacation.away.com/travel-do-beach-bali-sidwcmdev_115329.html. Not much you can do about the rain, I'm afraid, though I doubt it'll detract too much from being smack-dab in the middle of paradise (albeit with afternoon downpours).