Cooling salt water with minimal cost.

You could try to fabricate a heat pipe with stainless tubing.

en.wikipedia.org/.../Heat_pipe

Perhaps take a look at some of the mass concrete cooling setups and calculations.

It is going to depend on how large a volume and how much cooling is required.

You may be able to use plastic pipes to avoid salt water corrosion.

There's lots of data available on dealing with salt water corrosion but almost anything will add to the cost, such as using A4 (Marine) grade Stainless Steel. Using a heat-exchanger to increase the effective surface area and minimise the effects of any corrosion or other build-up such as salt coating the system. Plastic pipes may be resistant to corrosion but all the plastics used to make pipes are pretty goo thermal insulators so will tend to make the system very inefficient with regards to heat transfer which may or may not be an issue. A thin layer of paint, plenty of salt-resistant / marine grade paints are available, will allow you to use more convenient metal tubing. You also need to remember to consider the problems of deposition of calcium salts from the "cool" water. If you use a three-part system i.e. with a closed-loop between two tanks (one the salt water, one with the cooling mechanism) then that could be filled with de-ionised water (known as "distilled" but it hasn't been for decades!) then, depending on the rate of heat flow you need, you could just use a standard pond-pump or similar. Bob

A lot of your problems in this area have been solved by people making "keel cooling" systems for boats.

The applications are different but many of the components used might be suitable for your purpose.

This search page will get you to a selection of suppliers.

https://duckduckgo.com/?q=keel+cooling+components&t=ffab&atb=v101-1&iax=images&ia=images

MK

Suppose I needed to cool a length of approx 100 miles by 4 miles in shallow waters of 25 feet depth, would it be feasible to build something like that to cool down the water? 

I’m not familiar at all with this type of stuff. You said a 3 part system, is there any related information on how to build this. The  area is really large ( 100 miles by 4 miles) that will be needed for cooling, so it would have to be very economical to build and operate, if it is possible to do at all.

Thank you for responding and the info you have already provided. 
Erich 

What kind of insulation you have on top of that?

Geothermal is cheap and easy.

In the construction of the Hoover Dam, around 600miles of 1" diameter steel pipe was used for the mass concrete cooling process.

Before that, Fulgence Bienvenüe used similar cooling techniques to freeze the ground to stabilise it for tunnelling in the construction of the Paris Metro. I recall a similar ground freezing technique was used more recently for the Boston 'Big Dig' construction.

Some places use arrays of passive thermosyphon cooling piles to create permafrost conditions in ground to stabilise it without the need for cooling plants, but it requires sufficient temperature differentials for the syphon to work..