I've had fuel pumps, water pumps, electrical failures, shift cables, spun hubs, damaged props, drunken friends, blower explosions, all leave me stranded on the water but I have never had a total engine or drive failure. Volvo, Merc, GM, Ford, Bravo, OMC, Chryslers, Casale, Borg Warner, all of the big main components are designed to keep working. They almost ALWAYS fail due to lack of maintenance or damage caused by operator error. There's no shortage of owners here who will disagree and tell me stories of drives and motors that filled up with water and suffered catastrophic damage all by them selves. If a drive gets water in it it's nearly always because a seal or bellows was damaged and then the owner did not check the oil all season. That is not a drive failure, that is a maintenance error. Owners will also disagree with me by saying the shift stuff inside just broke. We all know shifting problems make themselves apparent and give you time to fix it before it lets go. The second I see water in my drives they are done. It's reseal time. Sure I've heard a few stories of blown drives only a few hours old, but I haven't seen them personally. I've had customers break shafts in drives because they didn't believe me when I told them the new engine they have can easily break it and then expect me to buy them a drive. This is why I no longer build for people outside my trusted base.
A friend just bought a really nice Sea Ray 240 Weekender with a supposedly new 350 in it and the salesman said it was a rebuilt drive. Turns out it was a saltwater boat, the upper gears in the drive had rust and pits in the bearing and race, the engine was probably replaced but the manifolds and risers were not new.
if his engine locks up and bends rods it's not gonna be the engines fault. If that drive top gear shreds it's a maintenance failure. The guys are exactly right. Whatever you get, take very good preventive care and you'll most likely get stranded by the myriad of other systems on the boat.
That's the problem with being a mechanic. We know all too well about flying machines in pieces on the ground and boats in pieces on the bottom of the lake. Checking the drive oil by loosening the drain plug and letting a few drops out after every boating day can save you grief. Loosening the bellows clamp and checking for water every time can save a large unexpected expense such as a divorce lawyer. Following a strict replacement regimen for exhaust manifolds and risers goes without saying, especially in sea water. Too many folks get a sense of security because they boat in freshwater and forget that the manifolds and risers are built by humans and assembled in the boat by humans using a very thin flimsy riser gasket. Boat transom assemblies, Gel coat, engines, drives, are all assembled by hand in the hulls and the potential for mistakes is astronomical compared to mass produced automobiles. For the huge expenses involved in owning and maintaining a boat there are way to many failures. From the factory to the dealer to the owner, all are touched by hands that are controlled by varying levels of intelligence and experience. Then add in the repair mechanic who's hands then touch it after something goes wrong and you can see how it is so confusing for a non mechanic owner who simply wants to go boating. Each and every human in that chain has his or her own opinion and point of view and it is a truly sad state of affairs for the marine service industry. Then add on all the invisible folks inside your computer telling you different things and it's very easy to see how our original poster simply cannot decide who to believe. For us mechanics it's pretty clear, but to the owners of a car, boat, plane, computer, it's like pulling teeth to get consistent answers.
Wanna hear failure stories? My biggest was a blower explosion which actually did very little damage to the engine but severely damaged my shorts. I think I can say the most failure prone component of the boat is the trailer.