I thought we had a voice line that noted it, but you can use the left/right arrow keys to switch, as well as num row 1/2/3/4 for specific turrets, or num pad 2/4/6/8 for specific turrets based on N/S/E/W.
I hear you on the enemies hitting hard, however-
We made sure that the game compensated for losing a turret, speeding up other turret fire rates and reload speeds.
Let me know if anything else comes to mind!