Belt comes off when wet. are there tensioner replacements?

I would think that either your belt is worn and stretched out or that the tensioner is worn and needs replacing.

Could be a stretched belt, or the tensioner may not be keeping it tight.
You should have plastic panels on the bottom of the van to keep water from hitting the belt.

Make sure that the splash shield is there and that the belt tensioner has the proper force on the belt; Check that all the pulleys are correctly aligned also.

check the belt and the tensioner. I would be the tensioner is frozen so the belt is loose. This will cause the light to come on when you start it cause the belt is kinda wet from condensation.

Replacing the belt tensioner and the serpentine belt will fix that for good. A temporary fix will be to remove the belt and wash it with regular soap and water inside and out (also wipe clean the pulleys), let it dry and apply Belt Dressing to the inside of the belt, and reinstall it. This will give the belt more traction and hopefully keep it in place.
I think your belt is worn and the belt tensioner lost some of it's tension, but, if you don't want to spend $$, try the above fix, it may work for you.
Good Luck!!
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the only water deflection dodge gave these cars was directly under the pully system if it has this and is still throwing belts check the tensioner

Sounds like a bad tensioner. Some cars have automatic tensioners and some are manual. If yours is manual, you just need to pull it tighter with a torque wrench. If it is an automatic tensioner, it will have to be replaced. Pretty easy, just unbolt the old one and bolt the new one on.

there is an updated tensioner pulley and hydraulic tensioner for this problem talk to dealer

Check all pulleys closely for warp or wobble, other than that I would look at replacing tensioner.

Certain brands (cheapo) of serpentine belt will do this. It is because the new belt is thinner than the old belt. This causes reduced spring tension by the belt tensioning device. Other possible problems are a bad belt tensioner, driving through deep water and getting the belt wet while driving or a defective crankshaft harmonic balancer assembly.
I HATE Dayco brand belts, because they do this a lot. I would have the shop inspect the parts mentioned above. If they are OK, tell them you want a new belt (for free).
Charlie