Shaun,

in the very simples sense though, if i had the same volume of water flowing
through both a very large and a very small outlet, the speed would be much
greater for the smaller outlet right?

The velocity (speed) of the water stream would be greater from the
smaller outlet. The resulting force, however, would be the same since
you're moving the same volume of water per unit time.

this seems like a way to achieve some
sort of gearing to me, despite whatever losses are incurred from
backpressure.

It's not a matter of backpressure, it's a matter of reaction mass. It
is Newtons second law of motion, paraphrased; for every action, there is
an equal and opposite reaction. The 'little' stream puts a lot of force
over a small area, whereas the 'big' stream puts a small amount of force
over a big area. In each case, the "force/unit area x area" quantity
(total Force) is the same. As long as the volume remains constant,
every increase in velocity will be offset by a proportional decrease in
the area over which it is applied.

It's not a matter of the water stream "pushing" against the water behind
the boat. Its just like how rocket thrusters work in a vacuum; you shoot
out 10kg of gas at 10m/s over a 10 second period, and you'll get exactly
that much "thrust" in the opposite direction. To be sure, there are
lots of hydrodynamic losses and effects for the boat, but the basic
properties of thrust are the same.

runing pumps in series would allow you to have a smaller
outlet and still maintain the same volume of flow right?

The same volume as what, a single pump with larger outlet? If you mean
use a second series pump to overcome all the frictional losses to
maintain flowrate, sure...but you're now powering 2 pumps. The cost of
the higher velocity, at the same volume, is all the additional power you
burn up in the second pump.

While there would obviously be a sweet spot for any given pump, having more
velocity at the outlet seems like it would probably result in more real
world 'thrust'.

The higher the velocity *at a given volumetric flow rate* the higher the
thrust. It's Newtons formula:

F = m x a

Where F = Force
m = mass (proportional to the volumetric flow rate)
a = acceleration (proportional to the velocity of the water leaving the
pump versus velocity entering the pump)

I was reading a page by an RC boat builder who use a bilge
pump for drive on his boat. he used a fishing scale to measure the trust
produced by the boat, and found that making the nozzle on the outlet
increased thust, but only to a certain point.

Yes, and that certain point is where the flowrate begins to decrease as
a result of the additional head pressure caused by restricting the
outlet. There are other issues that arise when the outlet is
sufficiently large that it represents a significant percentage of the
width of the boat, which you can do with an RC boat, that just don't
arise in 'real' boat applications.

Keith Hughes