Update
Finally took to boat up to the Yamaha dealer in San Antonio. Turns out there is low compression in all cylinders on the starboard engine, which I can believe. Apparently normal compression is 150psi and the cylinders are reading between 40-80psi. The dealer says the low compression is a result of corrosion, and is therefore likely NOT covered by the Yamaha Extended Warranty (YES). This I cannot believe.
Here is why I simply cannot accept that the low cylinder compression is a result of corrosion.
1. I'm OCD AF when it comes to cleaning my boat and the engines after every use. We typically use the boat two days a weekend during the summer months. We rinse the engines with fresh water every Saturday, and salt away every Sunday. We also rinse the entire boat after every use, and wash with boat wash every other Sunday. My boat is only four years old and in amazing shape.
2. There were zero symptoms leading up to the first time the engine would not fire up. If corrosion were the culprit there would have been symptoms leading up to the complete failure of compression in all cylinders. Misfiring, running rough, hard starts leading up to this, something, but there were none. On the day it failed to start, it had started up completely normally and ran like a champ initially. We even stopped and started the engines several times as we followed near a pod of dolphins. It was only after a several hours at the beach that it would not start, with the slow crank as the only noticeable symptom.
3. Slow cranking, as far as I know, is NOT a symptom of low compression. If there were only low compression due to corroded/leaking seals the engine should be cranking FASTER than normal, not slower. I'm no mechanic (at least not anymore, former helicopter mech turned pilot) and most of my knowledge of engines is related to turbine engines, but this isn't the most complicated principle to understand. Suck, squeeze, bang, blow. If compression is low, there is less resistance during the squeeze. Less resistance means faster reps and because air escapes the cylinder the mixture is too rich and does not ignite.
4. I pulled the spark plugs myself and did not see any corrosion on them whatsoever. I used to work on and now fly aircraft that spend a lot of time very close to the water. I know what corrosion looks like and what it can do, that's one of the reasons I am so very OCD about keeping my boat in tip top shape. The rep I spoke to was in service had not seen the speak plugs himself, but said his mech said there was corrosion on the spark plugs. I'm just not buying that. I'm not calling him a liar, but I'm guessing it's a common cause of low compression and I think he may have just said that when I started asking questions.
I'm sure there is more I am forgetting, but it's 4am and I am awake because this popped into my OCD mind at 3am and couldn't get off of it. Long story short, I am looking at a "short block" replacement, and it's going to get expensive if not covered by YES. I'm open to critiques to my line of thinking. I don't know much about timing, but from what I've read previously this screams broken timing chain to me. I do know that timing chains were a common point of failure on earlier model engines, but I thought it was something that Yamaha had addressed and corrected with a thicker chain.
More to follow as the situation develops.