Why do fish have backbones?

Skeletons are generally needed for structural support for land
organisms in order for their bodies to overcome gravity and aid in
locomotion.

What are the underlying structural and physical reasons for the
evolution of the endoskeleton in vertebrate fish, however?

In water there generally tends to be no need to generate a structural
support for the purpose of overcoming gravity alone.  The density of
biological tissue might be slightly greater than water, but swim
bladders or the like tend to work relatively well as a counterbalance
to those types of forces.

For locomotion, you need to have low drag or resistance to movement as
you are swimming through the water, but at the same time an organism
also needs to impart a backward force to the water itself in order to
propel that organism forward.  The generation of 'lift' would not
necessesarily be as critical to a water organism as it would be to an
air organism, again, because of the lesser density differential
between the organism itself and its surrounding medium, when an
organism is floating in water rather than when it is flying in air.

You generally tend to need a situation where the organism can generate
a high water resistance, impart a force, then generate a lower water
resistance in order to be impelled forward while simultaneously
extending itself back to its initial conditions so that it can impart
a new force, all without producing an opposite backward force of the
same magnitude, which would, ultimately, make it go nowhere if were
not minimised.

Now it just seems to me that fish tend to swim faster than jellyfish,
although I can't say for certain.

The early chordates were supposed to have a sessile form and a younger
moving form, that gradually became the fishes.

It would seem to me that a notochord or backbone would enable a
sessile organism to thrive in rapidly flowing waters withoug being
torn apart or rupturing.  Nonetheless, the notochord tends to be a
structure for the larval rather than the adult state for at least some
of the most primitave chordates, although I am not sure of the
relative extents.

Are there any structural reasons with underlying physics involved, as
to why a backbone or notochord might be advantageous for the purpose
of locomotion speeds in the early fishes, rather than simply having
opposing muscles resist contraction on the opposite side through
closed water chambers, or the like?

What advantage does a fish have, in comparison to a swimming worm, an
octopus, or a starfish?