How cold is really up to you and your physique.
Although 6 foot 4, i am a skinny runt. I swam across the forth late last year, as a result i know that i have a maximum of 35-40 minutes in water of 12 Degrees without any wet/dry suit.
As long as you are moving about, you don’t feel the cold. The danger is that though, i did not feel cold whilst i was swimming, as soon as i came out the water i realised how cold i was.
All i can say, is take your time and try to build up time spent in the water by 10mins at a time. I don’t know how much wet suites take the edge off the cold. But best to be safe than sorry in cold water.
Here is a couple of pictures. The more weight you have, the warmer you’ll be!
http://s54.photobucket.com/albums/g97/big_ginge21/?action=view¤t=Martin-Sunday.jpg
http://s54.photobucket.com/albums/g97/big_ginge21/?action=view¤t=dsc02921Medium.jpg
Cheers
M