Because the triathlon is a unique combination of events, functional training for triathlons should involve exercises from all major muscle groups. Functional training involves doing exercises that mimic the moves of your sport with additional resistance. The goal of functional training is to build muscle strength and endurance so your muscles learn how to drive you farther and faster during a competition. Pay attention to this type of training and you'll see your performance improve.
Functional training for triathlons should include exercises for a full range of motion. Pool running is one type of functional training that works well for triathlons, and it is also a great way to get a workout in if you are injured or faced with bad weather.
In general, pool running is one of the best cross-training activities that you can do. You can wear a specially designed belt that allows you to move your arms and legs just as if you were running on land. You want to stay vertical and put your legs through the same motion as running, rather than going horizontal and kicking as you would in a swimming workout. If you run in the pool for the same amount of time you'd spend running on land, you'll get an equivalent workout. Or, if you don't have a belt, you can use a kickboard.
To use a kickboard, place the kickboard under your chest and wrap your arms under it. Your feet will hang down into the water. Use a normal running motion to propel yourself forward. The kickboard version of pool running is less aerobic than using a belt because you're not engaging your upper body, but you will still work your leg muscles.