The biggest construction problem is the creation of something that
can be used as a piston/cylinder without a machine shop and expense.

What material to use and how to seal it, this is the issue.

Yes, the bag idea is good, but that material and how to seal it.

Rubber car/semi-truck tire inner tube and pvc pipe. This has limited
size selections, but one could use multiple piston/cylinders working together
during a single stroke to obtain greater volumes of work.

With unlimited resources and money, you build your idea, otherwise you
need a redesign of the idea to fit limited resources and money.

This means a more complex design due to less expense and limited resources..
One machined part might have to be replaced by multiple non-machined parts.

You ask yourself, what can I make that works?
Then redesign what does works into your idea.

Looking at this image:
How does the design change if the piston/cylinder in the image above
is not a sealed piston inside that cylinder.

Just creating any kind of piston/cylinder is a first step.
Water does not compress, and thus it will cause a leak and a
blow out faster and easier than a piston/cylinder using air.
A water leak generally is harder and takes longer to fix than an air leak.

Transfer of force from air to water is easy using water as your piston.

The stoke of a closed valveless air piston/cylinder, can be used to pump water via two one way valves.
The force of the air is transfered to the water and the water becomes the
piston and the water pipe becomes the cylinder.

See what I'm speaking of here.
I'm looking at a single direction, upwards only, power stoke.
The down stroke does do work, but very little compared to the up stroke.
The container holding up/down float is not closed and is open
This simplifies design requirements.