Electrical question on floating neutral generator for emergency use
Lets start with your premise that you must buy a 'floating neutral generator'.  That is simply not true, you can buy code compliant transfer switches with neutral-grounding disconnects built in as described in this video.

Next, you seem to be worried about a possible 'transient voltage;' created by double bonding your generator.  This will only occur if the generator is already self-bonded (bonded-neutral), and wouldn't be a concern in the scenario you described.

However, your concern about trying to power a water heater with a floating generator via extension cord isn't misplaced as water heaters often will not run if they detect a lack and unbounded neutral. Thus if you were to ever try to run the generator to power your water heater without it also being connected to the electrical panel for your furnace (such as having having your  your transfer switch turned off), you may experience an error.

To fully wrap you head around this, I found this video explained exactly what bonded-nutural is and the type of power a floating-neutral generator produces