dont you think the water will evaporate by morning?

I would cover the plants with something. Old sheets, blankets,etc. Sounds too chancey doing the water thing.

Something similar to eric30's technique is used in the Florida and California orange groves when temps threaten to be a bit below freezing. A thin layer of ice protects the trees.

I don't know the details (it's my dad, not me, who spent time on his grandparents' orange grove). And although the big local growers here spray the apple orchards in similar circumstances, I've no idea if the technique is useful with herbaceous plants.

Here's a bit on how it works:
http://www.ehow.com/how-does_5245655_spraying-frost-protect-fruit-freezing_.html

My friend 4 blocks away had his plants touched by frost and mine are still fine. My thinking is that by that late at night, it is cool and almost 100% humidity so there isn't much evaporation if at all. Plants stayed wet until the sun hit them in the morning. Water is a fabulous thing, it takes alot of energy for it to undergo a physical change. So far the 10 day forecast says that I'm safe through next weekend for now.

You could do one plant each of different types of frost protection and see what gives the best results.