HI! I'm not Bill but I'll give it a shot! The proper term for this is called "Sudden Stoppage". When a prop/rotor is suddenly slowed or stopped physics take place, sometimes very dramatically, and sometimes it is more subtle. Think pieces of shrapnel flying verses the engine losing RPM momentarily. I think the high Power/RPM=shrapnel picture is pretty much self-explanatory! However at low RPM, (revolutions per minute) the effects are much more subtle. But the physics still prevail.
First, if you're a smoker, grab a cigarette, If not, a empty paper towel roll will work.This would represent the crankshaft or propeller shaft of the engine. Next, hold one end steady and twist the other end. You will notice, your tube or cigarette has wrinkles in it. The end that you twisted would represent the engine crankshaft under load. The stationary end would represent the propeller where it attaches to the crankshaft. This wrinkling is call plastic deformation. It is normal and lends itself in a very practical way in everyday applications and is used in various formats in todays turboshaft aircraft to give the flightcrew indications of engine performance. This is all very well thought out by the engineers and is designed into the operational limitations of normal and emergency operations as it applies to aircraft powerplants and drivetrains. Now, Twist the tube to the point that the tube splits open and you have gross plastic deformation. This would be an example of sudden stoppage in the extreme, catastrophic Failure. Not good.
In the case of a radial engine, attached to that shaft are rods, pistons, and valve train components, throughout 360 degrees, all dependent on doing their thing in correlation to the crankshaft, in degrees of rotation. So twisting the crankshaft, beyond design limits, even just a few degrees in a millisecond throws this wonderful, orchestrated event into chaos. Kind'a like the mixing a beautiful waltz and slam dancing! With the expected results! And none of it good!
OK. Back to the less spectacular stuff. This crankshaft also drives "accessory" items that are driven by gears and shafts, Mounted on "Pads" or Common mounting points . All very critical to the various system that they support. This would include the starter/generator, engine oil pump,hydraulic pump and magneto. The latter required for ignition. These accessory items, back in the day, and still in some cases today, were designed with an intentional design flaw! The shafts that drive them were designed to shear or fail if they should seize (bearing/mechanical failure) or remain engaged when they shouldn't(starter) so they wouldn't cause a fire due to overheating or to prevent them from taking out another system. It is very possible that a "sudden stoppage" or abrupt deceleration in engine RPM could cause failure to anyone of these systems. The exception being the engine oil pump which is usually direct drive and does not have a "necked" or shear shaft.
So what would happen to the prop if a six or twelve inch wave hit the prop at idle? in my opinion, not a lot at flat pitch and idle. But, it would surely have got their attention! No doubt it would have resulted in a torque moment throughout the air frame. Would it cause a catastrophic failure in and of itself? In my opinion, No. Why? Propellers are designed to flex under load to a point and would have had the only damping effect besides gear lash on the engine/gearbox assembly. But the momentary shock may have caused secondary failure of some of the other systems as mentioned above due to the abrupt shock in the gear train.
Would this effect the starter? probably not, unless they were starting the engine when the wave hit. Could they hand prop the engine for a start? Sure.
Would it start? Most likely it would. But remember, one of those accessories was the magnetos. The sole source of ignition. Each engine had two independent mags for this reason.
Would there be abnormal vibrations? probably, but to what extent? That depends. The wave (only one! doubtful. I've never heard of a 12" rouge wave. I figure it would take a minimum of 20 seconds from cutting the fuel mixture to engine stoppage (rotation) . Not including reaction time.) striking the prop would have been only momentary, but the impact to the engine mounts could have been significant. They are ruggedly built but are not designed for that kind of abuse. Think about what happens when you throw a wet towel into a ceiling fan!(on a personal note: It's only funny when you do it the first time!) Also on the bulkhead that the engine mounts pass thru are where the cannon plugs connecting these various systems and there controls could in someway have been effected by movement of the engine mounts and torsional wrinkling of the sheet metal they mounted to or that they were grounded to. Accumulative effects.
As to your question of personal experience as to the effect of sea spray in close proximity to a prop would have, I'd have to say minimal effect. I've done a lot of low-level overwater flight, Military(<10') with nothing other than leaving streaks on the blades.
I do know, as one of the last accidents that I investigated be for leaving the US Army, a tail rotor strike on a 2 foot tall bush resulted in 2 serious injures and the loss of a aircraft. Sudden stoppage of anything important that rotates is not good!
I hope this has answered some of your questions. Hopefully it will provoke more. I just can't believe I got it done on one page!