Hi, great question - we are somewhat familiar with that style of water heater...whilst the Yamaha 1000w generator may be fine, often the information in the product manual is not enough and if possible we'd encourage you to contact the manufacturer (Duoetto) or the distributor and ask for clarification on the peak wattage draw of your particular model. Again, there are many 12V/240V water heater models that can be run easily with a 1000w generator, but its always advised to check with the manufacturer about the peak draw of your particular model to make sure you get the right sized generator...you dont want to buy a generator that isnt big enough to do the job you want.
Thanks, and good luck :)