It's directly beside a river.
You're absolutely right though, if you want big water, that's the way to get it. It's just that "big water" doesn't even begin to approach the scale of the water that would be necessary to directly fight this fire.
The real world is complex. It's not all like software or taking multiple choice exams. Again the devil is in the details. Pay really close attention to the map legend. It looks like there's nearly 200 feet of distance from the closest water shown on that map and the nearest side of the cathedral. I really doubt that all fire trucks carry 200 feet of hard suction hose, or have pumps designed to draft through 200 feet of hard hose.
"The NFPA 1901, Standard for Automotive Fire Apparatus – Requires pumpers to carry: 15 feet of large soft sleeve hose or 20 feet of hard suction hose"
In real life, if you were going to draft in an environment like this, (leaving aside any reference to fire boats for a moment), you'd drive an engine to within 20' or so of the water, have it draft, and then relay water to the fire scene using (usually) 5" LDH. But sometimes even getting an engine to within 20' of the water can be a challenge, based on the geography and circumstances.
If the body of water you're drafting from is (strongly) subject to tides, that adds another complicating factor as your water source is now moving around, which could require constant re-positioning of the apparatus.
Lay people always see fires near large bodies of water and think "Why is water supply a problem, they're surrounded by water?!?" But it's often more complicated than that.
Source: was a firefighter and firefighting instructor for about a decade in a former life.
My understanding, which is probably quite out of date, is that it's mostly harbor firefighting boats which had this ability.
A coworker of mine points out that many of the San Francisco manholes are actually covers for cisterns of water, which exist primarily for fighting fires after an earthquake possibly takes out the water mains and starts many fires. I should hope that San Francisco fire trucks can draft water to use these.
Park the truck next to the cistern. Connect a length of hose to the pump supply, drop the hose into the cistern. The pump will move water until it loses suction pressure.
London Fire Brigade for example has nine special pumps that can draw 2000 gallons a minute each from river or lake supplies. So that's 18,000 gallons a minute just from them.
And that's a smaller fire brigade than Paris.
SFFD trucks have that capability, and it was used in the 1989 earthquake. It's intended for pulling water from the underground cisterns marked by a big ring of bricks in SF intersections.
Terrible for the equipment to run salt water, but they had to.
For anyone else confused: this earthquake was in San Francisco and this is about their Fire Department. It doesn't mean that French fire trucks have that capability 30 years later.
There were notable logistical issues to get enough firefighters on-site given it’s an island, and there were a ton of people around when the fire broke out. They wound up having 400 firefighters on-site until midnight to get it under control and save the structure. I’m pretty confident they did all they could, and HN’s armchair firefighters aren’t likely to have done any better.