Difference in speed; head above water and head under water.

I have folowed a lifeguard course and we always had to swim with our head above water. 2 requirements for the exam were: - From the side jumping into the water, one lap entirely under water, 2 25 meter laps on back with arms above water (to prove that the kick is efficient enough) within 100 seconds. - 200 meters in 4 minutes and 15 seconds (which is even quite doable with the breaststroke), of course with head above water. At the university swimming pool I noticed a list for the sportmasterstudents (they study sport at an academic level) with required times and laps. This got me wondering. How much faster can you swim a lap when you swim with your head under water compared to when you swim with your had above water? I suspect that the relative difference in speed will be larger for a sprint than for an long distance.