When looking for faster swim times or a more efficient position in the water so that more of your energy goes toward forward propulsion start with the spot highest on top...the crown of your head.
The crown of your head is best described as the point on top of your skull. You can visualize it as that spot of intersection for a line that comes up from your shoulders through your ears met by a line drawn from the base of your skull by your neck which intersects with a line from your chin up through your nose over your forehead. If you ever wear a baseball style cap they often have a button on top that is the intersection of the various pieces of material forming the cap. That would be a similar approximation of this line.
Another way to see in real time is to stand in front of a mirror with your chin more or less parallel to the floor. If a board was balanced on top of your head the place of tangency would most likely be the crown as well. We call this head placement the "neutral" position.
When swimming freestyle - which is the stroke most of us use for training purposes - we want our head in this neutral position which means we lead with the crown of our head.
Anytime you lift your head up, even a half an inch, something opposite happens at the other end of your spine. Your hips drop accordingly. In the old days (1950's through the 70's) it was thought best to hold your head up with the water line at approximately your hair line. The idea was to get up on top of the water. Today this is not the case. You want your body to swim as flat in the water as possible to minimize resistance from your chest, belly and hips. The easiest way to do this is to drop your head, leading with the crown.
Initially it will feel like you are swimming downhill or underwater. This is not the case. It merely feels that way because what you are used to is swimming with your head held higher; in most cases too high. If you can see ahead of yourself in the pool then your head is probably too high. Look straight down at the bottom of the pool. You can practice this position standing in front of the mirror; simply get your head in the neutral position and then look straight ahead. If you were to rotate your entire body 90 degrees you would be swimming on your stomach in the correct body position in the water.
Give it a try and see how you feel. Once you get used to it, you will feel better; for sure your stroke will be more effective as you are now presenting a smaller amount of body resistance to the water.
Let us know how it goes!