Question
I just planted wave petunias in hanging baskets a week ago already they look their wilting and they are not dry

Answer
Check the roots.

Many a farmer knows about Diabrotica undecimpunctata, the yellow-and-black striped or spotted Cucumber Beetle, consumer of cantaloupes, cucumbers, zucchini, squash, watermelon, pumpkins and related members of the Gourds and Melons 'Cucurbit' Family.

But a beetle's gotta eat, and these ones will settle for Petunias.  Eggs hatch on the soil, then tunnel under, toward the roots, which they devour with delight.  Naturally, a plant without roots wilts, even when watered.  The beetles also dine on petals, like here:

http://bygl.osu.edu/mediamanager/node/5027

But maybe it's not a pest problem.  Too much water can lead to Root, Stem or Crown rot.  Leaves wilt; stems soften.  Gardeners panic and water more.

Verticillium Wilt is a real problem.  Do your Petunias look like these?

http://www.gardeningknowhow.com/plant-problems/disease/verticillium-wilt-treatme

Verticillium lives in the soil.  If THIS is diagnosed, you should throw them out and get rid of the soil they grew in.  

Ingrid Berlanger and Mary Powelson, plant disease experts at Oregon State University, post their advice on one species of Verticillium on the website of the American Phytopathological Society.  They make clear how tough this disease can be:

"Verticillium wilt is problematic in temperate areas of the world, especially in irrigated regions. The pathogens can persist in soil for many years in the absence of a susceptible crop. Infection is through the roots, and management of the disease is difficult."

They note that this disease thrives in moist soils with a temperature range of 70-81癋:

http://www.apsnet.org/edcenter/intropp/lessons/fungi/ascomycetes/Pages/Verticill

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Whatever the problem -- pest or rot -- you can improve the situation by using well draining, sandy soil -- not the kind that comes in bags from the supermarket or Home Depot.  Beetle larvae are injured by sharp sand particles and often succumb to those injuries.  

If pests are the problem, you can use the age-old method of crop rotation to keep them guessing.  Next year, don't plant Petunias there.  Plant something else instead -- Portulaca are drought-resistant and come in vivid colors.