another word of caution, thickness of wet suits is indeed measured in mm, but that is not the whole story. The construction of the wet suit will have as much to do with the warmth of use and ease of movement, as the thickness. For example my dry suit is 4mm thick, but is made of compressed neoprene, and is equivalent in temperature terms of a 7mm drysuit (but a lot easier to use). I had a 2mm wet suit that was constructed with a titanium mix, that was the equivalent of 4mm. When I started diving
I used a 5mm wet suit, but because it was a long john + top, the equivalent thickness was 10mm around the body.
Check out what the local diving shops recommend in the area you are diving. There might be a cold current
If you are travelling a reasonable distance in a rib
with a wet neoprene suit (wetsuit or a dry version) you will experience a considerable chill effect from the evaporation (I normally wear a windproof)
Get too cold underwater and you increase the chance of muscle cramp. get too warm and sweat a lot and you will also increase the chance of cramp, so this is an important area.