I'm talking about the strip of Kuta Beach from Hard Rock Hotel to the BeachWalk Mall/Sheraton Hotel.
If you swim in the middle of the day, the tide is in and the waves break too close to the shore - in other words, crap for body surfing.
The beach is quite clean, apart from the occasional plastic bag/sweet wrapper brushing past your legs. The beach graders and cleaners work on the sand early each morning.
If it rains, which it will in March (it's the end of the Wet Season), the beach is nigh on deserted in the middle of the day. No hassling, no sunburn, no glare off the water, no surf schools getting in your way. Very relaxing.
After 16:00 hr is a good time (as the tide is out).
Legian Beach (1km further north) is cleaner. Surf seems to be better in the afternoon, too. Doesn't break as close to the shore.