Some friends and i would like to poach a whole salmon, weighing four to five kilograms. There are two ways we could do this: in a fish kettle, or wrapped in aluminium foil in the oven (is that "en papillote" when it's a whole fish, wrapped tightly?).  I raised the possibility of using a dishwasher, but that was not well received.  What are the relative merits of these approaches?  Using foil means we don't need to find a kettle, and we don't have to wash it up afterwards, which are nice practical benefits.  But is there an effect on the actual cooking and eating? Does one take longer? Does one risk not cooking the fish properly? Does one make it harder to add herbs and so on? 
A whole salmon of 5 kg is going to be about 75 cm long. Very few, if any fish kettles are going to able to accommodate a fish of that size whole. If you find one big enough, it is likely to cost several hundred dollars (US), hundreds of GBP, or hundreds of Euros. Are you going to want to spend that large an amount of money in relation to how often you with use it?  The next problem you need to answer is, will a fish kettle of that size fit in your oven? I just measured mine, my oven won't accept anything much longer than 58 cm maybe  I would opt to cook it wrapped in aluminum foil. 