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The impeller in a car waterpump is METAL, for openers. Outboard
waterpumps have to work in a very different environment, and so they
are made of flexible materials. They are in contact with the insides
of the pump housing to maintain a needed seal, and the material also
can crack due to plain old aging and loss of flexibility. An outboard
impeller can deteriorate from just age, without having any hours on
it. That's why 2 years is a good time for routinely changing them even
if they have very low hours on them. Likewise, an event where a lot of
sand or mud gets sucked into it, or it is allowed to run while dry
will cause an early death. Running dry ruins them almost instantly.

They aren't ****. They are designed for a different set of rules than
a car water pump. Boats are not automobiles, nor are they airplanes.

An impeller of the type commonly used on outboards and small marine engines
can have too much flexibility. The vanes need to be quite stiff as, after
they have been pushed inwards by the cam, it is their inherent stiffness
that allows them to spring back quickly and thereby suck in more water.
I found this out when an impeller that seemed nice and flexible would pump
just fine at slow revs but would cease to pump at higher revs because the
vanes were too flexible to recover in the time available. I changed it for a
stiffer one and problem disappeared. I have never known one to crack and
think they would have to be out of use for a long time before this happened.
Do not forget that there is always one vane bent inward when the engine is
not in use so it pays to turn the engine over occasionally to move the
impeller around a bit.
Running dry is certainly sudden death to them and so I like to smear the
impeller with soluble oil (hand cleaner) when first starting unprimed in the
spring.