He made the statement that I should always turn off the raw water intake valve for a few seconds before turning off the engine.
Does this make sense? Why would this be necessary.
It can. It all depends. It is my understanding
(IOW: I am not authoritative on this subject) that, with certain systems, on certain boats, when the engine is shut off raw water can be siphoned from the exhaust and into the engine. This is a Very Bad thing. Closing the raw water intake a few seconds before shutting off the engine breaks the siphon and prevents this. The other thing you can do is install a raw water anti-siphon loop.
Here's a thread from a little earlier this year: raw water anti siphon/vented loop?
where they're discussed. I'm sure that if you search Sailnet and Google, you'll find plenty more on the subject.
The other thing to be aware of is being careful of cranking too much with the raw water intake open. The water lift muffler requires the exhaust from a running engine to empty it. If you crank too long w/o the engine starting, the water lift muffler soon fills up, then the water backs up into the engines exhaust manifold, thence, via the exhaust valves, into the cylinders and, eventually, just about everywhere else in the engine. Very bad.
If I have to crank more than 10-15 seconds, the raw water gets shut off.