Ruben - the cheaper the waders the more likely they aren't
breathable. The breathable waders are definately waterproof
but they have special material that allows the small water
vapor molecules that are formed from evaporation off your skin
to pass thru from inside to outside. At the same time this
special fabric keeps the larger liquid water molecules on the
outside outside. In this way you stay warm and dry on the
inside. If the day is a bit warm or you work up a sweat by
walking around in the surf the water vapor from your sweat
doesn't collect on the inside of the waders and the layer of
clothes that you have on stays dry. The ultra cheap waterproof
waders may trap the sweat vapor on the inside of the waders
allowing the condensation to collect in your socks or on your
pants. 

When the water is cold, I put on a layer of fleece on my legs
and I keep warm dry and toasty. If you go waterproof try and
make sure that the waterproof material is breathable. The
label should make the claim. I know Goretex is both waterproof
and breathable but it costs big bucks. There are other cheaper
waterproof and breathable material out there.

Richard