There are more restrictions on Personal Watercraft than two-stroke motors. Not all two strokes are banned either, only carbonated two-strokes without a CARB rating are banned.

REFER to this website:
https://dbw.parks.ca.gov/?page_id=28770

For the most part the following lakes require a CARB compliant motor. Diamond Valley Lake, Lake Skinner, Donner Lake, Lakes Tahoe, Cascade, Fallen Leaf, and Echo. Millerton Lake, San Pablo Reservoir, San Luis Reservoir, O'Neill Fore Bay, and Los Banos Creek Reservoir.

So about 12 lakes.

The concern is really about unburned fuel that contains the additive MTBE and that additive getting in to drinking water.