Should I add balloons in my Ultralight wing to help it float if I land in the water?
Not practical to put balloons in the wings.  They would expand at altitude, messing up the wing profile.  How would you control that?  

The best option, possible if the fabric is reasonably impermeable, is to seal all possible ingress points in the wing, leaving a small vent hole at a point least likely to be submersed, like the wing root trailing edge.  This will at least delay the sinking.   You could use ripstop repair tape used for fixing tents.

That'll slow down the sinking process, giving you lots of time to get out.  Then wear one of those compact inflatable vests that boaters use that drape around your neck that you inflate when needed, for swimming to shore.

Even better, with all the weight you would add with various other solutions, might as well get a set of Puddle Jumper amphibious floats.  Might as well make it possible to land on water anyway.  Problem solved.