Water in fuel tank, removal using meths

If you poor water into a running engine, it won't die, unless you get too much water...

If you get water in the fuel system, chances are it won't go into the engine though. It will sit in the bottom of the tank (petrol floats on water), in the metal pipe under the car, and depending on your type of carb, maybe in the bottom of the float chamber (HS carbs will probably suck it up).

Now, the reason water doesn't mix with petrol is that water is polar, and petrol isn't. Alcohol is a bit of both. If you mix petrol with water and shake, you get little balls of petrol and water which eventually settle with the water at the bottom and the petrol firmly on top. If you mix alcohol and water, or alcohol and petrol, you get a homogeneous mixture, you don't get any separation of the fluids.

Now, what if you mix some water, quite a lot of meths and petrol? The alcohol and the water join together (at the molecular level) and such groups can then mix with the petrol. So you get the water and the alcohol spread through the petrol, and it all gets drawn through the engine.

This isn't good for the engine but isn't particularly bad for it either. Alcohol is very bad for aluminium if it is left long enough, and in high enough concentrations. If you end up with a mayonnaise type slimy mess in the tank, then you didn't do it right, and probably need to add more meths, or a commercial dewatering agent. Even better, dump it and start again.