Since no others have answered...

swimmingworldmagazine.com/swim-cgi/ has tens if not hundreds of different workouts.

usaswimming.org "Tips and Technique" has interesting ideas on specific solutions. There was nothing specific to open water swimming.

A great source for a basic understanding of the various strokes is in the American Red Cross´s "Swimming and Water Safety Manual." (In the US, the Red Cross provides free or cheap swimming lessons. Volunteers are trained to use this manual to deliver the lessons.) Ch. 6 is the relevant part. Start at redcross.org, click to "Preparing & Getting Trained," then "Take a Class," then find the part for "Water Safety Instructors."

Re "drills," I often use the following (all for crawl): (1) one arm at a time ("catch-up" in the old parlance) to better monitor where elbow, forearm, hand are; often combined with (2) thumb drag along the side to ensure a bent arm recovery; (3) random, frequent flipping between front and back crawl to encourage sufficient roll; (4) swim with fists to "feel" whether the forearms are catching water; (5) count the number of strokes per 25m and try to reduce the number by lengthening the end of the pull. Sometimes (6) swim with eyes closed to determine whether swimming in a straight line. (I do wait til the lane is fairly empty...)