I grew up a wakeboarder and have a heater shirt which is pretty much a thin wetsuit top. Can I wear that for the swim portion of the triathlon? I have tri shorts that I will wear the entire race and I can just swap the heater shirt to my "regular" shirt with the number on it in T1.
Generally you can wear what you want. The shirt should not be a problem unless wetsuits are not allowed for the specific competition you will participate in.
yes, except keep the "normal shirt" on under the heater shirt, and pony up $5 for a number belt.