"if the capacitor is like a bucket, when the bucket is full why does touching metal across both handles make the bucket empty?"
water will be a bad analogy because it almost cannot be compressed.
Technicaly electricity is like gas (for example air). The difference between gas and electricity is that gas consists of molecules and electricity consists of charged particles (for example electrons). This electron-gas can circulate inside metal, but almost cannot leave it.
In order to leave metal, there is need very high pressure for electron-gas, because air or empty space are isolators - they like glass for usual gas. When electron gas leaves metal you can see it as electric lightning.
Then you can imagine two glass vessels connected by a hose. When air pressure in both vessels are equals, there is no gas movement in the hose and this is equivalent for 0V voltage between two leads.
Then attach the pump into the hose break and pump air from one vessel to another. You will have pressure difference between two vessels. This is equivalent for electric potential between two leads. Higher pressure difference between vessels is equivalent for higher voltage between leads.
Now your capacitor equivalent is charged. You can replace pump on the hose with air valve. If you open valve, gas will starts to move from vessel with high pressure into vessel with low pressure. This is equivalent for electric current flow.
Electric current in the metals is represented by free electrons (negative charged particle). These electrons moving from "-" lead to "+" lead. It happens because there is excess electrons on the "-" lead and lack of electrons on the "+" lead. When count of electrons on "-" lead and "+" lead will be equal, the electric current will be stopped. It means that your capacitor is discharged and now has 0V potential difference between leads.
Vessels volume is equivalent for capacitor capacitance. Higher volume vessels can accumulate more air molecules. Higher capacitance capacitor can accumulate more electrons.
This is how electricity works in classic metal. In salt water electric current flow works in a little different way, it consist of positive particles - ions. Ion is an atom or molecule with missing electron. Since it consists of proton which has positive charge and this proton charge is not balanced with electron pair, it has total positive charge.
The difference between metal and salt water is that metal has free electrons, which can flow between atoms, but cannot leave metal. And water has ions (proton charge is NOT balanced with electrons), which can flow between normal atoms (proton charge is balanced with electrons), but cannot leave water.
When electrons moving in the metal, it can randomly crash into atom, so the atom starts to moving, it leads to thermal loss. This is why metal is heating when electric current flows through it. The same thing happens in the air hose when it has some kind of air obstruction, when high pressure air flows through this air obstruction, it heating it.
In some cases it is possible to make condition for ordered motion of electrons with no collide with atoms. This is known as superconductor effect. There is no thermal loss in such case.
AC electric current is equivalent to rapid pumping of air from one vessel to another and back with high speed. It leads to cyclic air flow through hose back and forth. This air flow can be used for some useful work.