My truck was stuck in a river. Took out plugs and got the water out of the motor. As i was trying to start it (it started to run rough a few times) when we heard a loud snap and the motor locked, we backed the flywheel a couple of turns and tried it again and something punched a hole in the oil pan. is it possible to fix this without replacing the motor?

You have thrown a rod, the connecting rod for the piston to the crankshaft, this will require that the engine be removed from the truck, the condition of the crankshaft, and block will determine if the engine block itself needs to be replaced. This can not be fixed with the engine in the vehicle, you may just want to find a good used engine and replace it, hope this helps, let me know

What you Have Described is Indicative of Throwing a ROD. Practically the ONLY thing this Can Be Is A Connecting rod Broke Loose and was Ejected Into the Oil PAN this Is FATAL Engine Damage.
To Fix it Without Replacing the Engine ? YES But Engine Must be removed and Disassembled to repair it and the COST Would Be Excessive.
Best Bet is Replace Engine, New Engines Have GOOD WARRENTEE"S --- Used Engines have SHORT Guaranties BUT With a SKILLED Technician Doing the Selecting for you along with Finding one With LOW MILLAGE then you get off Much Cheaper that NEW One. Hope This Helps and Sorry to Have BAD NEWS on Your Engine

You can replace the connecting rod you broke without removing the engine IF you have no damage to the cylinder wall or main webbing Cylinder walls must be measured to detect any bulges (rod broke due to hydrolock) It's likely that the big end of the rod is still intact so crank may be OK.
First remove pan...most damage will be on the bottom. If no damage seen other than the rod, remove the cylinder head & check bore. It is wise to check all rods for bent condition if they too ingested water. If all is good replace piston and rod assembly and put it back together. Or, get a different engine, put it in and repair the broken one when you have the time

It is possible to repair the engine in the vehicle if the pan can be removed and inspection of the block passes. The problem though is that the crankshaft cannot possibly have survived this catastrophe, and the engine or the transmission must be removed in order to remove the crankshaft for replacement. If you miss any detail the engine may fail again and be all for naught. If it were me, I would be finding another engine. Consider also: the transmission, rear axle, brakes, fuel system, power steering (if equipped) all need purged in order to avoid finding yourself stranded in the near future.