∴ The rated current of the water heater I=PU=2000W220V≈9. 1A, and the rated current of the multi-function socket is 8A, which means that the rated current of the water heater is greater than that of the multi-function socket, which will cause electricity accidents;

The water heater is a metal shell, and it should be grounded with a three-pin plug.

So the answers are as follows: ① Socket: the current allowed to pass is less than the current when the water heater works normally; ② Plug: The two-pin plug cannot be grounded.