Swimming in the pool to train provides different opportunities for speed, endurance, and technique training because it's a mostly-controlled environment where you can focus on a specific aspect of your workout without too many other variables. You might have a pace clock or use your watch for a guaranteed amount of distance, and have the flexibility to use training tools like paddles or buoys when you need because they're readily available at the end of your lane.
In open water, conditions are not consistent and for most, the distance isn't guaranteed to be accurate for comparison every time. Even if you swim the same loop of a lake, it likely will change in some way, whether it's from sighting, water temps, environmental conditions, or something else. And, if you're anything like me, you might zone out/lose focus in open water a little more, focusing on what everyone else is doing, a cute fish below you, the sun, or anything else that catches your attention.
Without walls and a pace clock, training in open water takes a different type of workout with a little creativity. Here are some ideas for how to incorporate different types of workouts into your distance training.
Distances: if you know general point-to-point or loop distances, you can create reps out of them. If you know a point-to-point swim is 500 yards, you can use pool workouts to incorporate a build, descend, or technique element into that distance. For instance, your workout may be 4 x 500: first is warm up, second is build to 80% through the 500, the third is 80%+ pace consistently, the fourth is warm down/recovery. You can stop between reps for short breaks by treading water.
Time: if distances are unknown and you don't have tech to tell you when you've reached a distance, you can use a simple watch to reach a duration of a rep. For instance: 6 x 5 minute swims, descend 1-3, 4-6, with you keeping track of the effort expended on each 5 minute swim to keep yourself continuing the set. It's totally fine to stop between reps for a short rest and tread water.
Number of strokes: alternating focus by counting to a number of strokes, like 100 strokes easy, 50 strokes super power pull, 50 strokes easy, 100 strokes super power pull, can shake up your workout and give you something else to focus on.
The chasing game: pretending you're chasing someone or something, or being chased. You can choose a random buoy in your local swim spot and race toward, especially effective if it's another swimmer that's swimming away from you.
Technique shakeups: using the distance, time, or stroke counting to switch from drill to drill so you're able to keep getting more efficient outside the pool. Focusing on long reaches, powerful catch, body position, kicking, and smooth sighting for specific periods during your workout will help you build new habits in open water, rather than just the pool.
Environment toss up: choosing to swim in friskier conditions can help you build stamina and discover little muscles you never knew you had! If the conditions are safe, but potentially more turbulent/not ideal, like light rain, slightly windy conditions or a stronger current, choosing to get in, even for a short workout, will help you adapt your stroke to varied conditions. If you're nervous, swimming with a paddler can be a great way to build confidence and acclimate to various conditions.
Switch from swimming: you can still get a workout in doing other activities on the open water, like kayaking, canoeing, and prone boarding. Adding core strength from paddling, or upper back and neck stamina from prone boarding can be a very effective way to improve your open water stroke.
Tow your gear: you can add resistance by towing more weight in a tow float, like the Quackpacker, or swimming with a paddleboard or kayak attached to your tether. It's a great way to strength train, especially if you have to do a shorter workout that day. The Quackpacker can hold 30+ lbs in it's storage compartment and provide mild resistance with added weight.
Remember that vertical kicking, whether flutter or dolphin, no arms and trying to stay as high out of the water as possible is great for kicking muscles, core muscles, anaerobic exercise, and keeping warm when waiting for others