I sell Webasto AC units, I have 5 and 9k BTU units in stock, PM if you would like a quote.

Big boats, Fast boats and now AC?

What you would need to do an engine driven Ac system are:

Compressor, belt & bracket. Evaporator & condensor. You will also need fans to move the air over the evaporator & condensor. One needs to be cooled & one will need to blow the cool air in to you. You would need to make up custom lines & you will need a pressure switch for the compressor to keep the ac line pressures in check. Oh yea you will need some 134a refridgerant too.. You could probably search the web & build this ala carte.. Jamie / Lakeside

I would just use a small (5000btu) self contained marine unit and run it off the batteries with an inverter. You may need to add a battery or go up in size for the ones you have but if you are going to use it while running you will have plenty of juice from the alternators to run it. The only thing it will need is a constant supply of water.

I have a mermaid marine 5000btu unit in the cabin of my 28 daytona and it will run for more than 8hrs off the batteries with an inverter (engines off). The whole setup was less than $2000 and was easy to install. The unit only weighs about 30 lbs. PM me if you want.

Has anyone come up with a recipe for cooling the cockpit with an engine driven system?

I would think a vintage air system with a modified heat exchanger to use raw water from the engine system in place of the air cooled automotive one would be pretty easy to make work. I plan to build a custom system at some point

I did just find this company for coolers...
packless.com

I'm going to investigate if between vintage air and packless to come up with cockpit ac, heat, and defrost...