Is it permitted to land amphibian plane in the ocean on Californian coast?
Seaplane air taxis only work where the logistics situation and market can support a business case for that kind of service.  It's an expensive way to travel, so there has to be some kind of benefit that makes it worth the cost to the traveler.  

This works in the Seattle area and the Lower Mainland of British Columbia where you have large city centers with nearby destination locations that require over-water travel to get to, and business travelers willing to pay extra for fast waterfront-to-waterfront travel.

Vancouver BC has a thriving air taxi market because people want to get across to Vancouver Island quickly, especially Victoria BC, the provincial capital, and the business traveler demand supports seaplane operations up to Twin Otter size.

California? Way cheaper to drive, or go taxi-airport-taxi.  In Vancouver BC or Seattle, a seaplane air taxi service is competing with ferry boat service so it has a serious leg up that makes the ticket price worthwhile. Not so much down south on a contiguous coastline where there are both land and air alternatives.

So bottom line is, there's no money to be made doing it.