If you say its mega smooth but you cough my guess is that your taking to big a rip, personally I find I can take huge rips ona smooth bong, i can suck till i go blue, but if its to big a rip on the exhale i'l cough, so i recon thats what it is just take smaller rips.
Aint ever heard of water vapour making people cough, do you cough when you have a steamy shower?