How many times have you offered the helm to a guest (or inexperienced
boater) when having to attend to something down below, especially in open
water?

Here is a real life story that should give you some caution when doing so.

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From Boat US Magazine, March 2005:

Guests who lack experience should be monitored constantly whenever they are
at the helm. That means you -- the boat's skipper -- should remain on deck
to act as a lookout. There are many accounts of boats that slammed into
other boats, jetties and shoals at the hands of an inexperienced guest who
had been left alone at the helm.

One of the more unusual Boat US Marine Insurance claims involved a guest who
took the helm one night on a lake in New York. Before the skipper went
below, he told the guest to "head for the red light", referring to a light
marking a channel some distance away. Instead, the guest headed for a red
light that, as it turned out, was on the back of a train. The boat bounced
off a rock jetty and wound up high and dry on the railroad tracks. A few
minutes later a second train came tearing down the tracks and slammed into
the boat."
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Doh! I bet that left a mark.