Scenario 4: Short from hot to neutral or grounded body of an external device:

Under this scenario for all cases you will be dependant on the breaker. If the current is high but the breaker is slow on acting there might be a fire. The use of an Arc Fault Circuit Interrupter instead of a breaker will reduce the fire possibility.

Scenario 5: Short from Neutral to water:As usually the neutral wire is grounded either at the outside were the wires enter the
house
or at the main nothing happens unless this neutral grounding is not perfect some small voltage may develop creating the following situation:

a) No GFCI and No Probe: nothing will happen, you may detect small voltage of the water to ground, if you touch the water you may feel a small shock if you are not insulated enough.

b) GFCI but no ground probe: if you touch the water and the voltage is high enough to generate at least 5 milliamps the GFCI will trip.

c) Ground probe but no GFCI: The small voltage may create a permanent small current, this may be enough to start killing the most sensitive critters, (Podsand Snails) and create coral RTN over long term.

d) GFCI and Ground Probe: same as 5b above. If the current is below 5 milliamps it may still create a small current which may affect the tank inhabitants but in a lesser degree given the smaller current level than 5c above.

So in summary overall the safest for the aquarist will be the
installation
of AFCI, GFCI and Ground Probe. The safest for the inhabitants will be AFCI and GFCI without ground probe. This is the reason why many aquarist do not use ground probes (but will always use GFCIs) specially traveling ones that stay away for extended periods of time. If you have a remote alarm system installed that may page you or send you an E-Mail in case of a fault (Like the aqua controller from Neptune Systems) you shall have no doubts and have your Probe installed with your GFCI's)

Opposite the most dangerous situation is not having at least a GFCI. By the way many states, city or county electrical codes require installation of GFCIs on any location where electrical equipment may be exposed to water like outdoors,
kitchens
, bathrooms, aquariums, spas etc. If this is the case and there is an accident or fire the insurance company MAY NOT PAY. so be warned.