There are several issues with the EPB system on these trucks. If it is making noise, you know the one, when trying to apply, it's one of two reasons, either the cables are seized up, or the actuator is out of adjustment to the brake shoes.
Do not continue to try to apply it. It will strip the gears, and it will need everything.
With it being recently after being submerged, I would bet that the shoes got dirty, dragged on the drums, and the heat caused the cable sheathing to melt, and seize up. When you tried to apply it, the actuator wasn't able to pull the cable, hence the noise from gear slippage.
The cables can be replaced separately, however, until the job is complete, there is no way to know if the gears in the actuator are still in good enough shape to work. Personally I recommend replacing the whole unit, and inspecting the brake shoes and drums (rear rotors) in a situation like this.
There have been several service bulletins on this subject, including the one that says the 'fix' for the noise is a 'unjam' procedure. This will only work if the reason for it making noise is due to it pulling too much on the cable, and not actually tightening, due to incorrect brake shoe adjustment, usually in this case, it will sound somewhat normal for a second, and then make the screeching noise.