if it wasn't making the tapping noise before you went through the water, then you can pretty much rule out your valves being out of adjustment. is the knocking REALLY loud? sounds like someone beating on the bottom of your car? your idea of loud and ours may be a bit different. you need to try and isolate where that noise is coming from- top of the motor(valves) or bottom(the expensive side)

 

Sounds like you bent a valve or threw the piston out of wack. That cold water commin in on a hot metal part can realy warp the crust outta stuff... Dont drive to farm from home and keep a cell with you... Buy a 4x4 and that problem disapears on the street, but gets worse in the local mud pit....