If you can handle the mid 50s for an hour then you can go without a thermal shirt. I go year round without that stuff and it drops as low as 47F here in S.F., it is a process of acclimation and I would advise using the 10% rule. By that I mean to incrementally stay in longer than you did your previous time (i.e. last time 30mins, next 35mins). Whatever you decide to do, please take AWP's advice and regarding getting warm immediately upon exiting the water. Remember your core body temp will continue to drop after you have left the water. Good luck and stay in the cold water its awesome!