There are really only two types that meet that requirement. The Honda at over £1000 or the many copies at around half the price. They are really only suitable for occasional use on the boat at anchor or moored up. They have significant downsides - noisy (cheap copies usually noisier!), use petrol and have to be operated in the open air. They are an inefficient way of producing 240v for heavy usage, although many find them useful for occasional use such as running power tools and appliances when laid up. Whether it is worth the hassle (heavy, bulky, need to carry petrol) when cruising is really a personal choice. Some, such as long term liveaboards who want to be more self sufficient clearly do even if their neighbours in a quiet anchorage might not agree.
Many think it better to spend the money on enhancing their 12v systems with larger battery capacity and alternative charging methods such as solar, fitting an inverter and accept that they can't use 240v for heavy use unless on shorepower. With a properly set up system there is no reason why you should ever have a flat engine start battery, and you can further reduce the already tiny possibility by wiring the system so that you can use the house bank for engine start in an emergency. Having both banks flattened in normal use is serious mismanagement.
In the situation you describe where you are only going to use the generator when moored to a pontoon without electricity then there is some sense in having a generator for power tools, using a kettle or microwave while on board. This is what many of our club members did before electricity was installed on the pontoons. the generator was kept on the pontoon while it was running and no petrol (or even the generator) stored aboard. In this case one of the cheap copies is probably the better choice.
