I work with a log of triathletes who are always working on the balancing where they spend their time on the three disciplines. In that balance, swimming is usually the place where the least amount of time is spent. I would like to talk to those tris about getting over the hump.
I like to describe it as a speed hump in the pool. If you never get in shape enough to get over the hump it will push you backwards all season. If you, however, you get in shape enough during the early season, you can then lean back on it for the season. This does not mean you can skip swimming for the season, but you will be able to retain the speed increase for several months. I regularly have swimmers drop five seconds per hundred yards after completing three weeks of base building.
Many a tri-athlete is on break, or just back from break. Once you climb back in the pool, start by getting in some baseline swimming. Feel your stroke, and get back to your normal workouts. This can take three weeks or more. Once you are near your workout normal then start upping your yardage. For a minimum of three-weeks increase your frequency to a no less than four swims a week. At the same time increase the distance of your main set, while keeping it on your base interval. Staying on a short rest interval is an important part of building your distance. Don’t drop the interval for a rest interval.
Using this base-building concept, you will get a strong aerobic swimming base you can ride for several months of swimming only two-to-three days a week. I suggest doing this twice during your preseason training and then backing off to spend more time on running and biking, as well as in the weight room and on core.