While others (like Marin) are NOT comfortable in a single-engine boat but are comfortable in a single-engine airplane or automobile. People are weird.
The Grand Banks we chartered before buying our own boat was a single. I was fine with it. I have no issues with single engine boats in terms of reliability or maneuvering. I am totally "comfortable" in a single engine boat.
However, I don't like them. And the reason I don't like them is operating them is dead boring to me. I like operating equipment, and the more complex that equipment is the better I like the challenge of learning and mastering its operation.
When we decided to buy our own boat we didn't care if it was a single or a twin. The boat we'd been chartering was a single, the canal boats we'd started running in England and Wales are singles, and this was all just fine.
As it turned out, the boat that best met our requirements and what we wanted to spend on a boat was a twin. I was a little apprehensive when we got it as I'd never run a twin engine boat before. A good friend--- the founder of Kenmore Air--- who had a twin engine, steel-hull deFever, told me in response to my expressing my apprehension to simply, "Go out and start using it. You'll figure it out just fine."
Which is what happened. And I also learned hat I REALLY liked operating a boat with multiple engines (our newest boat has three which is even
more fun).
So my preference for multiple engines has nothing to do with being comfortable or feeling safer, it's because running a multi-engine boat is
really cool.
The fact we don't have to come home on the end of an expensive rope if an engine has to be shut down is a great bonus, but it's not the reason I prefer multi-engine boats.
My wife, however, has stated that she is more confident in a boat with more than one engine under the floor. And a happy wife makes for a happy boating experience. But she has owned single engine planes and has never balked at the idea of flying into rough country in a single engine plane. Why she is fine with one engine in a plane but doesn't want one engine in a boat is something she's never been able to explain.
The reason we fly a single engine floatplane is that it's the only choice unless we want to cough up major, major bucks for a Twin Otter on floats. Floatplanes work best as high wing planes for reasons too numerous to go into here. Low wing planes have been put on floats from the Piper Aztec all the way up to the DC-3. But from a practical point of view, they aren't. So the world of floatplanes you can actually do something with like haul lots of stuff (as opposed to kit planes, ultralights, etc.) is pretty exclusively limited to single-engine, high-wing planes. Most multi-engine planes are low wing.
While I got a few hours flying a Cessna 310 in Hawaii, I never got a twin rating because I've never needed it for the flying I've been doing for the last four-plus decades. And even back in the 1970s, twin-engine planes were expensive to fly and even more expensive to own. I had a ball flying the 310 for the same reason I have a ball running multi-engine boats. But the need and the dollar justification have never been there.
I never got a helicopter rating for the same reason. I have one hour in a helicopter as part of a project for work many years ago. All I did was try to learn to hover a few feet off the ground. I have to say it is the hardest thing in terms of operating a piece of machinery I've ever done in my life. I'm sure I could have mastered it eventually, but I can tell you that anyone who thinks flying (manually) a helicopter is easy is full of crap. Sure, one can learn to fly one very, very proficiently. But in comparison to flying a helicopter, flying a fixed wing aircraft is like lying on a sofa taking a nap.