One of the secrets of good front crawl swimming is for your body to sit high in the water, kicking with straight legs with your torso, hips, knees and feet all in one horizontal line parallel with the surface, as Michelle is demonstrated here:
This position creates minimum drag so you can move effectively through the water.
Low sinking legs hugely increases your frontal area in the water, slowly you down and making the stroke feel like a real struggle:
There are quite a number of technical aspects that can have an effect on your body position but if you have low lying legs then a good place to start is to do a little work on your leg kick technique.
Note that we’re not actually looking to generate much forward propulsion from your legs. In fact, most adult swimmers should be looking to generate nearly all their propulsion from their arm stroke. Instead, we are simply looking to reduce effort and drag to lift you higher in the water as you swim.
The most important aspect of a good flutter leg kick is to make sure that you are kicking from the hips rather than from the knees. If you have done much cycling or running then you might be in the habit of bending from the knee as you kick:
This uses a lot of oxygen and energy (increasing your need for air) and adds drag, sinking the legs low in the water.
Instead we should keep the legs fairly straight and the toes pointed, with the kicking action coming from the hips:
Don’t lock your knees rigidly straight, there should be a little softness in the knees but make sure the kicking movement is driven from the hip.
A great drill to work on this is “Ballet Leg Kick” which you can perform holding onto the side in the deep end of your pool:
When you return to full-stroke swimming, think about keep your legs long and straight with the big toes brushing as they pass. Do you feel higher in the water? Have a friend or life-guard give you some feedback on how deep your legs look. Bringing your legs up higher (even a little bit) will immediately makes things feel easier and improve your speed.