Let's take a look at a simple hip-flexor stretch that could really help improve your swimming. Many swimmer's legs drag low in the water creating lots of drag, slowing them down dramatically:
A low body position is very characteristic of the Arnie swim type but can affect any swimmer to a greater or lesser extent. If you're much faster in a wetsuit or with a pull-buoy between your legs, then a low body position is likely to be the single biggest thing holding you back with your swimming.
Unfortunately there's no silver-bullet solution to improving your body position in the water, it requires an all-round approach developing all aspects of your stroke technique. However, if your hip flexor muscles are tight then this will make improving your body position very difficult and will hold you back significantly.
Your hip flexors are the muscle group at the front of your hip which contract to lift your leg upwards from a standing position. In water, if your hip flexors are short they will want to contract at the hip and so draw your legs forward and downward in the water:
To develop the length of your hip flexors, use a towel or some form of cushioning under your knee and position the other leg out in the front of you with that knee bent at around 90 degrees:
Hold your upper body tall and strong, and gently press the hips forwards. You should feel the stretch at the front of the hip and possibly down into your quads. Don't force it but hold the stretch for at least 30 seconds on each leg.
As with all stretches, makes sure you are properly warmed up before starting or you risk a muscular strain or tear. Don't rush the lengthening process, it will take many weeks and months of regular stretching to gradually lengthen out the hip flexors. Take a "little and often" approach here.
How do they get short in the first place? Modern desk jobs have us sitting for many hours with the hip flexors in a shortened position and cycling (especially on the tri-bars) works the hip flexors whilst in a shortened position. This is why triathletes are especially susceptible to this problem.
If you have low lying legs in the water then you'll really benefit from adding this simple but powerful stretch into your routine. If you have no other spare time then practise it in front of the TV in the evening when the kids are in bed!