If you will be using the boat every say once a week is not necessary.
However, if you use the boat once every month you need to have it out of the water and wash everything out.
I do this as a matter of cause every single time during the winter time with my boat. I wash everything, polish everything, grease everything, check everything. Every time I get back it takes me 2 hours every time to clean it.
In the summer is different since the boat is in the water all the time and I use it every single day.
Although this states the obvious, the reason you should wash the engine out is to get rid of any salt deposits from the tubes and also wash out the salt from the impeller. I use the ear muff things but I also use a barrel with a slight solution of washing up liquid to rinse completely every out.
I know that is taking it to the extremes though