Back in the 60's when I was a kid, the TV station in Pittsburgh ( KDKA) , sent out a plea for help for the wildlife in the Mountains of Pennsylvania.
They gathered up mountains of hay, corn, oats etc and tried to feed the deer in such places as Potter County and the Allegheny National Forest - where the snow got 4 - 6 feet deep that winter. They even dropped it in by helicopter.
It was found that the deer did not fare well, like others has said because of what their main diet consists of and because of the organisms in their stomach could digest. They actually froze to death with full bellies of hay.
After that, the PGC took down the corn crib they had at the end of the road near the SGL 31.
They ended their winter feeding program and we actually ended up with more deer than what they had when they did feed them.
It would be better to just plant a crop and then leave it in the field then to try to feed them something different in the middle of the winter - just because it is cold outside and they look hungry.
I have seen deer eat pine needles, mountain laurel leaves, shrubs in peoples front yards and all sorts of weird things like tomato's. A deer will only eat what is available to them at the time. Which means that if you didn't plant the watermelons - they would just move onto something else.
The other thing that I thought about is the fact that a deer has to go to water at least one time per a day. Even better is two times per a day. Running water is better than standing water - but then again they will drink what ever is available. If no water was available - I guess they would go after watermelons if they knew that there was water inside of them.
It's just all a matter of getting what they need to survive out of what the land has to offer.
Winter feeding is the worst thing that you could do. Even if you did manage to feed the deer, after a while they would get used to being fed and change their feeding pattern and pretty soon they would get hit by a car crossing the road to get to your place and in the end it would still be one less deer.
I know - I picked one up along the road the other night coming from my aunts house - while delivering her Christmas presents.
It was only 50 yards from the peoples swing set in the back yard and only 25 yards from their apple tree.