That's the beauty of Yamaha 4-strokes... These days I run a single Yamaha 4-stroke on my boat (as does virtually every commercial crabber on the Chesapeake Bay where I live) and I've been boating for over 30 years. I've absolutely no problem taking my boat 15 miles offshore with my single Yamaha as, properly maintained, they are virtually bulletproof. Heck, it's not uncommon for commercial crabbers and local Coast Guard in my parts to rack up 7,000 to 10,000 hours without a single issue. These aren't the outboards from the past - these modern outboards are as dependable as the engine in your car (your car only has one engine). These days, twin engines just means about 60% more weight on the transom, 60% more money spent and about 35%-40% more fuel burn for the same performance a single high HP 4-stroke provides.
For me, if my Yamaha ever did break down on the water that's what my radio and towboat US insurance is for. In Cozumel... Let's see... There's only about 100 other dive boats at any given time floating within 2 miles of shore that will offer a tow. I assume if a dive boat with a single engine experiences an engine failure an Aldora Captain and any Captain from any dive op would offer a tow back after he picked up his dive group but maybe I'm wrong and if I am, those dive ops need to find new Captains.