A lot of people are confused as to why there was insufficient water to fully fight these fires.
The number one reason is we didn't have the infrastructure. When you have people living in semi remote areas at the top of hills, it costs a lot of money to run high pressure water pipes out there. Had these houses been on fire separately, one at a time there would have been plenty of water to fight it each and every one. But when you open all the fire hydrants in a neighborhood or region, water pressure drops especially at the top of hills where all these buildings are. The current through the water main line that feeds the region gets so high that the fluid friction through the pipe becomes the limiting factor. You would need to install much larger pipes all the way back to the gravity feed reservoir that you are relying on.
On top of hills the place that is hardest to fight fires. Fire goes uphill, water wants to go the opposite way. Strips of houses built at the top of naturally vegetated hills are extremely vulnerable to fire and extremely difficult to engineer water to. How much more do you want to increase your utility bills or taxes to make sure that the extra expensive houses on the top of those hills are well protected?
There have been some reports that power outages also contributed to the problem , but I think that's less likely because pumping typically occurs during the filling of reservoirs that are uphill from the users. The final pressure is derived from a gravity feed from the reservoir not from continuous pumping. This is necessary because water is incompressible and it's very difficult to maintain constant water pressure from a direct pump, there needs to be some sort of gravity reservoir or air pressure reservoir stage to buffer the pressure.
Related to this, it's completely unfeasible to pump water from the ocean to put out these fires. It's not just the disaster psychological effects of increasing soil salinity in those areas. Pumping would require a huge amount of sophisticated infrastructure and the introduction of whole salt water water towers or reservoirs.