Luke, I assume you paid someone to fit the impeller? In which case I would take it back to them and let them fix it. Whilst Davie (and others) advice on tracking down the problem is probably useful for someone who has experience - if you weren't confident about changing the impeller you really don't want to be debugging someone else's mistakes. If it is genuinely not pumping water - it wont take much time playing around trying to fix it before you completely ruin the engine. If it is cutting out too after a service then I would also be asking questions. A good marine engineer would happily take it back and run it up to show you if all is well when it warms up, but be knowledgable enough to deal with any problem before it damages itself.