Yes, anything you cover the ground with will slow natural evaporation
some. To fix the excess moisture there are several options depending
on your specific site characteristics. You need a knowledgeable
person experieced in your area to come out and look at your site and
give you options and costs. The visit should be free.

The same with your patio. Type of patio material and site
characteristics will determine cost. Ask around and get references on
a good landscaper, you should be able to get one company to do both
jobs. I wouldnt tie myself down to contractors referenced out by
large chain stores. Few good landscape contractors buy their
materials there.

Good luck,
Bryan