Unidirectional vs Bidirectional TVS diode for ESD protection UART bus
Unidirectional voltage suppressor diode with clamping voltage greater than 3.3 V is sufficient. For the positive spikes, the ESD diode will kick in into action soon after the voltage reaches the clamping voltage. For negative spike, the TVS diode acts as normal diode shorting the signal soon after the forward voltage 
of the diode is crossed.
SMF series from little fuse has reverse stand off voltage starting from 5 V. 
General purpose ESD protection diode example is below:
SP3222 0.9pF 30kV dual channel TVS for example can be used.



Adding a small capacitor between the line and the ground also helps.
Also consider adding a series resistor of a few 100 ohms depending on the baud rate of the UART. This will also helps in limiting the current value during an ESD strike. 
The resistor and capacitor value should be such that the 5*RC should be atleast 10 times smaller than the bit rate of the UART communication.