If you shower under way you don't need to run the b****y engine to get hot water!
Please check if moored to a lower craft and you decide you will run the engine (please see above) that the exhaust is nowhere near them....if it is, DON'T!
Sorry, rant over.
Now, yes, I agree with all the advice above, and we will always help newbies as much as they ask for, they are supposed to be enjoying a holiday and it can mar their enjoyment if they are worried about boating.
one other point: Think ahead. When planning to moor. Get your ropes laid along the deck where crew will be standing, in their Lifejackets ready to step off.
Crew Have a system of hand signals to the helm if s/he can't hear over the engine, and if you are the helm, look out for what the crew is doing. They can help if you are reversing but only if you are aware of them.