Cut off two thirds of the plant. Since you suspect over watering, let them dry out. Can you get them out of the sun? Also, since you may have watered them too much, perhaps you have leached out the nutrients and they need a little food. First, let them dry out some. Then, give them a little liquid fertilizer. Miracle Grow or whatever. It's probably overwatering and loss of nutrients.
Keep us posted.

Thank you, eden, for reading my post and for making a recommendation.

I have already transplanted a portion to a small pot and to fresh, dry soil. So, I will water and feed with liquis fertilizer tomorrow morning.

I don't see how you could have overwatered them....mine are planted bare-root in the rocks around my pond with their roots totally submerged in water all the time. they get HUGE and lush every year, even in full sun. they do wilt down during the hottest part of the day when full sun is hitting them though, but perk back up as soon as the sun goes away.