What you are suggesting with a back-feed cord is possible, but not recommend for a bunch of reasons, including the ones you mentioned.
The first thing that comes to mind in terms of simplicity is replacing the power wire on the water heater with a cord made for a dryer and plug it in to the generator. When your power crisis is over, you can either wire the hot water heater back, or install an outlet.
The next best thing to do is to buy a transfer switch sub-panel, install it, and move the circuits you want to keep using to that sub-panel.
I will join longneck, you need to give to give this one to a pro.
If you do not understand about back-feeding a circuit, already, it is too easy to kill someone. I say this without exaggeration.
The biggest problem with running a generator with a transfer switch is that you have to wire it into your main panel.
For basic protection of life, (as well as the law,) it must NOT be possible to have your generator and the power to your house on the same wire, at the same time.
This is not a case of, just for a few seconds, or I would not do that, or I will triple check a written list.
It must not be possible under mechanically, electrically, on with quantum physics.
The reason is that if you put power on the 'dead' wires to your house, it is possible to kill someone a mile away, or more.
Emergency workers expect down lines to be dead. Yes, they do check first, but that does not mean that the situation can not change while they are working on them.
Transformers work backwards, low voltage at your generator (240) quickly becomes high voltage (thousands).
And sometimes people, and pets, or cars, just do not even see a fallen power line.
This is a case of some money to a pro vs life.
Best Answer
Most often generator feeds in residential applications are now done with kits specific to your panel that backfeeds your panel through a breaker near the main breaker, and has an interlock bracket that prevents the backfeed breaker from being turned on unless the main is switched off. The generator breaker is then wired to an inverted (male) receptacle to cord connect to your generators 240v receptacle.
Any or all the circuits in your panel are then effectively connected to the generator, and you need to select which circuits in your panel you wish to turn on or off to prevent overloading and tripping the breaker on the generator. A generator with a 240v/50A receptacle will often allow operation of almost everything, some coordination of air conditioning, electric ovens and electric water heaters would be necessary.
If air conditioning isn't a must-have then I would advise considering a smaller generator. The one you have selected uses a gallon of gas an hour at half load, and runs at 72db. The sound level may have HOA repercussions.