First things first:

If your camera has gotten wet inside, turn it off and remove the battery IMMEDIATELY! The worst damage from getting compact electronics, such as cameras, wet with water that does not contain salt is from stray voltage shorting through the impurities in the water and damaging the microscopic electrictronics. With salt water, in addition to shorting things out, the salt will cause massive corrosion to the internal electrical components if it gets inside the body. Cameras that have been salt water damaged are generally considered unrepairable.

Don't put the battery back in until the camera has been thoroughly dried inside and out. This will probably take spending several days in a specialized environment with moisture absorbing materials.

Why could cause this? I assume the light is routed a different way when using the viewfinder and maybe it is going through something with water/condensation on it?

You are correct that when you use Live View the camera meters differently than when shooting via the viewfinder. The exposure meter used when shooting through the viewfinder is a dedicated unit located up in the prism housing. When you're using Live View, the dedicated light meter is not receiving any light because the mirror is flipped up. In that case the camera measures the amount of light detected directly on the main imaging sensor.