If your race is in open water, make an effort to get to a beach with a lifeguard for your second workout of the week. Inform the lifeguard you are practicing for your first triathlon; ask them to keep an eye on you.
Swim about the same amount of time in open water that has been normal in your previous workouts. Feel free to relax and take your time. If possible, swim to a buoy that is around 25 yards out, then swim back. Rest as much as you need between each "outing." If you are not swimming in open water on race day or do not have access to a safe open-water swimming situation, Workout 18 is a repeat of Workout 17.
Week 10 - Workouts 19 and 20: After warming up with a few yards at the pool, swim 3 x 200 yards with 15 seconds rest between each 200. In total, swim up to 30 minutes. For Workout 20, swim about 15 to 30 minutes in open water or repeat Workout 19 in the pool.
Week 11 - Workouts 21 and 22: After your own warm-up, swim 500 yards nonstop at the pool. Make the odd-numbered 25-yard-segments (a single length of the pool, unless it's a 50-meter long pool) very easy and the even-numbered 25-yard-segments faster.
For Workout 22, repeat Workout 21 or swim up to 30 minutes in open water.
Week 12 - Workouts 23 and 24: After a good warm-up at the pool, swim 500 yards nonstop. Make the first 100 yards of your swim very easy. On the second 100, try to relax your arms on the out-of-water portion (recovery). For the third 100 yards, focus on a strong hand pull. On the fourth 100 yards use a strong underwater pull and relaxed recovery. Finally, make each 25-yard length a bit faster on the final 100 yards.
This is the same strategy to use during the swim on race day. For Workout 24, begin with a short warm-up, then swim 250 yards at race pace. Include four accelerations consisting of 12 powerful arm strokes during the swim, whenever the mood strikes you—this is known as "fartlek" swimming. Take at least 24 strokes of easy swimming to recover after each acceleration. After a short cool-down, get out of the pool.
Since just about every triathlon is on the weekend, an option is to eliminate Workout 24 and just rest pre-race. If you are using this plan as a stand-alone plan (and not training for a specific race) you can do replace Workout 24 with a short warm-up and a 500-yard time trial.
Even if you are currently a non-swimmer, in just 12 weeks you can build the endurance to swim for up to 30 minutes. At minimum, you'll be ready to swim 500 yards. If you hit the pool now, it won't be long before you are on the start line of a sprint-distance triathlon!