CASTRO VALLEY, Calif.—California's relentless drought is beginning to dry up revenue in its popular lake and river tourism industry. Marinas and boat ramps across the state are turning away customers, and even spots where water is still relatively plentiful say visitors are staying away, assuming things are worse than they are.

Casualties abound from a three-year dry spell that also has wreaked havoc on California agriculture and prompted mandatory statewide restrictions on urban watering effective Aug. 1.

Los Angeles County officials have banned swimming in summer getaway Castaic Lake, citing dangers such as unexpected drop-offs in a lake now less than half-full. Boaters there report new hazards, too.

"There are humps and underwater gravel bars coming up out of nowhere," said Steve Goodman, a 31-year-old aerospace engineer from Castaic, Calif., who fishes there regularly.

At Don Pedro Lake near Modesto, Calif., which stands at about 45% capacity, the Moccasin Point Marina closed its boat-servicing operations in June due to low water levels.