What actually changes
For about three weeks in March, downtown Cabo San Lucas and Medano Beach fill with US university students. The bar strip runs later and louder, Medano Beach has music from competing sound systems, and boat party operators run a different product than they do the rest of the year.
Hotel rates in Cabo San Lucas rise. Restaurant availability tightens. Police presence increases. None of this extends more than a few miles.
Where it does not reach
San José del Cabo, 45 minutes east, runs a normal March. The Tourist Corridor resorts are gated and self-contained. Todos Santos and the East Cape are unaffected entirely.
The boat trips, the whale watching and the snorkel tours all still run normally — they leave from the marina in the morning, before the town's late-night rhythm applies.
The April alternative
April has the same zero rainfall, a warmer 86°F day, the same 72°F sea, and the first two weeks still fall inside whale season. Rates drop meaningfully after the first week.
The only date to check is Semana Santa, the Mexican Easter holiday week, when domestic travel fills the beaches. It moves each year.