There is probably a State code for pool barriers. If you found it in the UBC it was probably more in the direction of a public pool and not a private pool. The requirements in the IRC are in the appendices so if a municipality didn't adopt the appendices then there is not a code that covers them...unless a specific ordinance was adopted.

I do know that the great State of Texas has had swimming pool barriers as a requirement under the Health and Safety code since 1993...or even before but that is the earliest I can find easily!

I'm sure California had something in place before Texas ever thought about it!