Radar as the defining naval development of the 1930s

What was the most important development in naval warfare during the 1930s?

Radar, with the 1930s identified as its entry into service rather than the beginning of the underlying technology.

Radar is the choice here for the most important naval-warfare development of the 1930s. Its technical origins predate the decade, while its entry into service makes it the relevant choice for this comparison.