Difficulty in staying afloat. Both are easy to fix. Get your bearings. Use a line of tiles or lights or other markings on the ceiling to help you set a straight course. Failing that, just hug the lane line.
Most pools have a set of colorful pennants hanging across the pool near each end wall.
Get balanced on your back. On your back, you keep your butt from sinking by leaning on your shoulder blades and the back of your head. For a more efficient stroke, keep your head still, but roll your body from side to side as your stroke. As one arm comes overhead in the 'recovery,' that shoulder should nearly touch your chin. Following these simple techniques will keep your body sleek and balanced as you cut through the waves like a pro, relaxing as you go.