these are good for milder cold water, 65 and up. they are pretty hot to wear but this can be remedied by getting yourself wet as you need.
i think hydroskin works well as an insulating underlayer. in fact, if you have gone in for a swim hydroskin can be chilly by itself when wet.
for serious cold water, especially less than 60, i would think wet suit or dry suit.
Bought these for the cool water on the jet ski. They fit great and are just the ticket to keep my lower extremities warm while being easy to take on and off.
I love Hydroskin - it's a perfect weight for spring and fall watersports when you are in and out of the water. Unlike the tops, I found that I did not have to buy a size up in the pants. They are very easy to put on...it's only .5mm and is relatively breathable, so overheating is not a huge concern unless it is really hot out. The legs of the pants are really long...seems like a 34" inseam, but it's not a big deal and the stretch of the fabric takes care of this.
purchased a large size - wear 34 waist / 32 length and this thing is a beast to get on. i have no problem getting into my neoprene wetsuit but near impossible to get into these pants. once in, it's very uncomfortable and binding, especially for sitting position in the kayak. upsizing "may" help.