Broken valve spring

Valve springs are made from Steel, the steel is a special blend and the wire is rolled out into lengths, they study it for defects as it is rolled out and most times they catch the defects before it is made into springs. If a defect gets in an engine it will eventually fail.
The length of steel rolled out is determined by a detected defect. The steel is monitored electronically as it rolls out and most times defects are caught. You just had bad luck.
If this is an old engine I would change the one spring, if it was recently rebuilt with all new springs I would change all 16 springs. I say this because if one is bad the whole batch may be bad.
The only other time I see broken springs is from over revving an engine.
When over revved the springs may coil bind, open till the coils are collapsed this can fracture a spring.
On a HD Truck engine over rev might be 6,000 RPM, on a Race engine over rev might be 10,000 RPM springs are purpose built and have limits.
Over Revving in a boat is not common since the prop limits RPM with constant load. Were you jumping wakes in this boat at full throttle with the prop coming out of water? Not likely.

With my being in the metal stamping business for 20 years, I've seem plenty of broken springs. Failures were always due to exceeding the life of the spring or more common was exceeding the compressed length limit. A material defect will show up in relatively a short period of time which I feel is not the case with Bruce's engine. Spring wire whether round or square is made under very tight quality controls. Defects typically show up during the wire drawing process.