Many triathletes who do not come from a swimming background (which is most likely 90% of the triathlon population) dread hearing the race director announcing that the swim will not be wetsuit legal.
Like a school kid praying for a snow day, triathletes pray for a wetsuit legal race resorting to mystical beliefs like wearing their wetsuit inside out and dancing around their hotel room in hopes of pleasing the tri gods lowering the temps of the water. It is as if the whole race will be ruined without a wetsuit.
So how do you bounce back from the decision that you will not be able to wear one? The key is to not let it affect you. 
It is true that wetsuits will make you faster especially for weaker swimmers by realigning your body position, raising your hips, and preventing your legs from dragging.
How much time will you save by wearing one? It varies but if I had to give a rough estimate it would be around 2-5 for an Olympic, 5-8 minutes for a half, and up to 10 or even 15min for a full–more if you are in saltwater. You can see these times are significant but it will not destroy your race unless you let it.
The first step is to realize that you practice without a wetsuit, so racing without one will be just like practice.
Fortunately for you, everyone cannot wear a suit so everyone’s time is going to be affected equally. It is out of your control, so accept it, move on, and focus on what you can control like your swim stroke and breathing pattern.
Moreover, the silver lining is that you will not have to deal with chafing and your T1 time will be much faster.
Others are less concerned about the time lost and more concerned about safety; wetsuits prevent you from drowning. While a legitimate concern, new swimming precautions have made the water safer than any other part of the race.
If you are afraid of drowning or something happening to you in the water, remind yourself that you are in good hands with lifeguards and volunteers lining every meter of the course. Swim related deaths are very rare in tri and rarely if ever have to do with wetsuit legality.
Even though the water may not be cold enough for a wetsuit, you still might feel cold. If you are like me and get cold easily, then I recommend double capping and wearing two swim caps instead of just the one provided by the race.
Overall, focus on the controllables and regardless of whether you have a wetsuit or not, you have trained hard for this day so make the best of it.
