Do I need a thick wetsuit to stay warm?
No. Tempting though it is to clad yourself in as much rubber as possible before you immerse your body in cold water, a wetsuit which is 5mm all over will not necessarily keep you warmer.
Yes, the thicker the neoprene the greater the level of insulation but when that comes at a cost. The thicker the material the more your body is restricted in its movement, the more tired you get swimming and therefore the more likely you are to feel cold.
Thicker suits also give extra buoyancy which you might think is a plus but not if you want to swim breaststroke or prefer a more neutral body position for front crawl.
The open water swimming /triathlon wetsuits we stock at Tri Wetsuit Hire are all specifically designed for swimming which is as important for beginners as much as it is for seasoned swimmers. Swim specific suits are generally made up of lots of different panels of different thicknesses and flexibility. So, for example they could have 5mm neoprene on the legs and 1mm around the shoulders. This difference in panelling also means that you can get a much better fit than if the suit was 5mm all over. A thinner well fitting suit is better than a thick ill fitting suit.
If you do feel the cold, the better option is to use a thermal wetsuit which has a thermal lining or use accessories such as thermal vests, hat, socks and gloves. You can buy or there hire accessories too!