My first reaction to this is to ask why the motor is greasy to
begin with. Is this a used motor you bought, or did you buy it
new and it's gotten greasy since you've been using it?

I just bought the used boat one month ago. Actually, the outboard
motor is cleaner than my car engine. I am not sure whether I really
need to clean it at all. But the book on boat maintenance suggests
that I should clean the boat as a course of standard maintenance, and I
really cannot judge whether this is an overkill or not.

The main risk in hosing down the motor is getting water into
the electrical stuff. The air intake should be big enough that
you can cover it with plastic wrap or something, but the
electrical things involve little crevices where wires & sensors
connect. Take care with the direction in which you aim
the water, and use a mist.

I understand the part about plastic wrapping the air intake, and use
misting instead of hosing down the motor. I don't understand how you
will clean the electrical things and wires and sensors. How do you
clean them? Do you clean them with paper towel or just leave them
coated with oil and grease?

Do this early on a warm day if possible so things
will dry. Even better if you can get the thing warmed
by the sun first, or even run it for a few minutes so
the block is medium warm (but not really hot).

Sound like this is not something I will do in this year because I am
expecting warm but rainy days in this week, and there will not be any
more warm days ahead. I will clean the motor next summer when the
weather is hot and dry.

I don't like the idea of coating the engine with anything afterward.
Anything on the motor will inhibit cooling.

This means you prefer cleaning the oil and grease from the motor, and
you don't want to add anything back afterward. Good. This makes sense
to me.

Jay Chan