I have just had a chat with the race referee and she has sent me the following.
Swim Length Forbidden Above Mandatory Below
Up to 1500 m 22º C 14º C
1501 to 3000m 23º C 16º C
3001 to 4000m 24º C 16º C
If it is marginal then let the full race wear wetsuits but you can take the air temperature into account.
I am down the Lake today and will do another water test.