Replacing props underwater. Lesson learned?

I'm not a material science engineer, but I find the stress marks on the shaft fascinating, as well as the discoloration on the outer edge of the shaft. To my untrained eye, it looks like the metal fatigue was on-going for sometime.

That has nothing to do with replacing the prop underwater. That has to do with either vibration issues for a long time or weak metallurgy. It broke clean far in front of the keyway on a solid piece of shaft. If it had to do with the prop being loose, it would've snapped at the key way.

I'm with Skippy J on this, That was not a propeller installation issue.
In the pictures the key way is perfect, no distortion of shaft material here.

Also, that is an old crack.
That shaft hit something a while ago, maybe years.

I assume you have 2 piece shafts? Spare on board? Maybe a dive team already scheduled to replace the shaft?
Great work finding the wheel.

That is a classic propeller fit / key way induced shaft break. The crack most likely propagated from the keyway, the break was clean and happened very fast once it began to fail.

I really disagree.
In the picture of the prop with the shaft end still attached, The key way was barely cut into the shaft at the break. There is no deformation or pattern around the key way.
You can see the C patterns across the shaft from the splinter.

Run it by Frank and Jimmies or some other prop shop, the fact that it is right at the end of the key way cut says it all. I have seen this exact pattern on many occasions, always at the most forward end of the keyway cut-out.
The stress riser most liked occurred at one of the corners of the keyway, could have been from a past grounding or the poor fit of replacing a prop,
A material defect in the exact same spot as the keyway? Just randomly? I don’t think so.

Are we looking at the same picture?

My guess is the prop didn't get all the way onto the taper by binding on the key. The half moons radiating from the edge opposite the splinter broken off suggest impact vibration causing the damage. Very likely, the telling evidence will be on the shaft metal inside the prop. If the shaft stub just drops out when the nuts are removed, that would tell the whole story.

Did anyone notice the ding / repairs on the blades?

It's a Lazzara. Lazzara has always built their boats without enough blade tip to hull clearance as well as putting the underwater exhaust too close to the propellers. Getting on plane with lazzara's, if you don't advance the throttles ever so gingerly induces caviation that feels like you're driving over railroad ties at 50 mph throughout the entire boat. Has nothing to do with the prop or how the prop was installed, it's forward of the keyway. If you look at the severed piece that sticks out beyond the clean break in the shaft, the s/s shaft material is very porous. This is a cavitation issue over years and years, many cycles of whipping the shaft, and most likely a prop that was off/damaged prior to this one, and subsequent stress cycles. I think the prop had a log or other similar shaped floating debris get between prop and strut as you can see one blade has a lot of damge very close to the hub...... I know of several Lazzara's that have flung the prop and shaft off in the same area. A keyway break usually results in the shaft split in half across lenghways. Lazzara's are also famous for having breaking and having to change motor mounts way too early.

Just like a fell tree. A lil abuse on the left who knows when (last week, 3 years ago), them TIMBER.
A diver nor key way cut caused that.

The fact that they were able to locate and retrieve the lost wheel - That's the headline!

Yep, You notice 40' off line and when it landed, it's spinning/flying when it found the bottom near covered a blade? A week later.
Another few days and it may have been lost.

Another thread on YF a few months ago, the owner could not find his old wheel.

Hours after one of my customers lost a wheel at the Riviera Beach bridge (yes, many years ago it was a draw bridge), I found his 26" wheel barely. A blade tip was just showing.
It screwed itself down nicely.

The vast majority of these breaks are due to a previous grounding incident, soft or hard.
Cavitation is not the cause nor is whip, as the shaft was sufficiently supported by the cutlass bearing. 10% prop clearance is done all the time, not a factor on the shaft breakage.

Shaft diameter size when compared to horsepower and reduction gear, safety factor, and material selection combined with a physical impact all have a role to play in this failure. If we knew the hp, rpm, shaft diameter, shaft material, could easily calculate the safety factor which has to be above 5.0.

Other factors - was the prop barrel a full length SAE size in relationship to the shaft diameter and propeller diameter.

The clear break exactly atthe exact location forward of the cut key - not a random event.

Have you ever been on a Lazzara when they're trying to get on plane and the amount of vibration they have? I did a seatrial on a 68' where BOTH surveyors came running out of the engine room and screamed ALL STOP as the motors were vibrating so bad they thought they were going to tear the stringers right out of the boat. They didn't realize it was a common Lazzara deal and if you try to come on plane other than really slowly it whips the shafts something crazy, and has torn several engines right off of the mounts on several of them..........Lazzara came up with an add modification on the hull to try to shift the exhaust gasses elsewhere which helps some but is not an end all to end alls. The engines vibrate so much they literally look like you're running a prop with 3 majorly bent blades at cruise speed. It is something fierce. Every Lazzara regardless of size that I've been has exhibited this, including Pascals (which I've never been on).

I'm not sure you have studied the pictures either.
BUT, The main O P concern, Did a diver install the propeller wrong causing this failure.
I say NO and NOT the cause for this failure.