The advantages of singles and twins can change depending on where one boats. For example, I gather from posts to this forum that vessel assist coverage along the ICW is pretty to very good. So from the aspect of having to shut an engine down, having one engine does not seem as problematical as it can be somewhere else.
Where we boat there is a maze of islands pretty much all the way from the lower end of Puget Sound up through SE Alaska. Add to this an increasingly great tidal range as one moves north--- perhaps 10-12 feet here in the Sound into the 20-foot plus range up north--- and the currents in this region can get pretty impressive, particularly in the narrower passes and channels.
Roll in the increasing scarcity to non-existence of vessel assist organizations as one moves north and what might be an inconvenient engine shutdown along the ICW can become a downright dangerous event out here.
I know boaters here in the southern waters who've come within a hair's breadth of losing their boat when it became a race as to which would happen first--- help would arrive or the current would sweep them into the rocks. And a fair number of boat have been on the losing end of this race over the years.
Farther north one is increasingly on their own when it comes to emergencies. The good news is that on the commonly used routes along the Passage there is a reasonable amount of boat traffic, particularly during the summer boating season. So help may be fairly close at hand.
But off of those routes or in the off-seasons one is more often than not totally on their own. Combine this with the strong currents, narrow passes, and almost totally rock geography the risk of a very bad outcome from an engine shutdown is not something to be taken lightly.
A common argument in favor of single-engine boats is that commercial fishing boats are generally single engine. Totally true, and it's been this way since they started putting engines in fishboats. But.... these boats are mostly crewed, particularly with the larger seiners, gillnetters, crabbers and combination boats used today. They tend to have at least one crew member who is a very good mechanic. And they have the space and capacity to carry a lot of spares and tools.
So there's a bit of a difference between the single engine fishboats that work this area and the average recreational cruisers one sees up here.
My point is not that smart boaters should only have a multi-engine boat in these waters. There may be some people who feel this way but I'm not one of them. Considering the huge number of single engine recreational boats, power and sail, that ply these waters without incident year after year it would be a foolish position to take.
But it is something to keep in mind when determining what kind of boat one wants for cruising this area, particularly if one is interested in venturing farther north and off the more beaten paths.
Based on our direct observation over the last 17 years of cruising up here we do not put as much faith in the "protected prop" advantage of a single engine boat as other people might. If we count sailboats, we know more people who have fouled or damaged their running gear in single engine boats than we know people who've had the same problem with a twin. And some of these people ended up in serious situations when it happened thanks to the conditions I described earlier.
Now we also know a few people who've fouled or damaged running gear in their twins. But in all these cases, only one side was affected. So they were able to continue home or to a port for repairs on the other side. Only one of them ended up in a potential boat loss situation and that was because a log jammed the rudders hard over in addition to bending a shaft. So they had power on one side only plus they didn't have rudder control.
The couple were in the process of abandoning their deFever which was only yards from being swept into a cliff by the increasing current when the log popped free on its own and they got rudder control back. They returned the several hundred miles to Seattle on the other engine without further incident.
