When you harvest sweet potatoes, put them in a cool area and let them sit for 2 weeks or more before eating, I do not wash them until I am ready to use, just brush the dirt from them and store them.
Ours did not do well this year either and we have them on a daily watering sprinkler.
Oh cool, yeah basically that is what I did.
Hmm well I would think they would do well with lots of water.. If I had to pick a plant I can consistently grow well its sweet potatoes.
Maybe the water will not hurt if you get them good and dry before storing them.
I did a quick search and this is what it says
To help your sweet potatoes last longer it's important to avoid washing them before storing them. Sweet potatoes last the longest when stored in a dark place for up to 6 months. Do not keep your sweet potatoes near other vegetables as they can cause other foods to ripen more quickly.
I also saw that you should handle them as little as possible.