I don't want to be too picky, but this is an important safety topic. A GFCI DOES "interrupt" the current when it is triggered, but it has nothing to do with how MUCH current is flowing. That is the job of the main breaker in the panel (of course, it is only designed to protect the WIRING, not YOU).
The operation of a GFCI is actually really simple. Basically, it measures the current flowing through the hot terminal AND the current flowing through the neutral terminal. These values are fed to an analog comparator. If these current flows differ by a predetermined value (milliamp range), the comparator triggers and the device opens (aka: "interrupts") the current flow on the hot (and possibly the neutral) circuit to stop the current flow. Because it uses electronic means to test and interrupt the current flow, it is VERY fast and it shuts down well before you are harmed.
There is on other common misunderstanding on GFCIs: they do NOT require a ground circuit to operate! In fact, NEC recommends installing GFCIs in older houses built without grounded outlets that can't easily be upgraded (remember, ground is ONLY a safety circuit, nothing more). Anything that causes the amount of current flowing between the hot and the neutral terminals to be different enough will cause the GFCI to trip, so they are an acceptable replacement for a safety ground line.
For example, your submersible pump has failed insulation and the windings are in contact with the water. There is 300mA of current flowing into the pump and 300mA of current flowing out of the pump, but the water has been brought up to "hot" potential. The GFCI does not trip at this point, as the currents are still in balance! Now, you come along and reach into the tank to right a snail that your Queen Angle has pulled off the glass (AGAIN!). You are bare foot and standing on a slightly conductive surface, so when your fingers touch the water, 50mA of current starts to flow through you and you get a good tingle!
Before you can even THINK about pulling your hand back (before you even realize there is a problem actually), the GFCI measures 300mA at the hot terminal going INTO the pump, but only measures 250mA coming back FROM the pump and BAM! It trips and interrupts the current flow. It does this in microseconds, possibly saving your life in the process. Good little devices!
And, just to be complete, after you fix your problems use the excellent advice above, INSTALL GFCIs!!!!! If you had to be pulled out of the tank, you came WAY TOO CLOSE to being another statistic!