Is it easy to find fishing charters by walking the docks and bartering with the captians. What should I expect the cost to be for 3 people on a 28ft boat fishing for Marlin for a full day with out drinks or lunch included?

It would be hard NOT to find captains and sailors alike selling you the service on the docks, you will be overwhelm by the number. You will be ask to go see the boat, they will show you pictures, and you can bargain all day long, prices will range from 230 to 500 depending on the boat, and your bargain patience, Be sure they include the license and some might allow that you buy your own lunch in the near by restaurant or stores.  Just take a walk around the Marina, and shop, it is full of all kinds of boats and packages. No wonders why Cabo is the Marlin Capital of the world.