I misunderstood this when I watched the film. Why were they required to fly low the entire trip over the channel, over Holland, then to Germany etc? I thought it was to fly under the radar but when the flights approached the Dutch coast, someone mentioned that they would be picked up on radar now.
Flying low, close to the water, would delay the time until German radar picked them up. However, sooner or later, they would be spotted. Generally, the closer they were to the ground, or the water, it was more difficult for the radar operators to pick them up from the ground echo.
As for low flying all the way there and back, that's fully explained in the book. Gibson learned about it from Micky Martin (who flew "P" for "Popsie" on the raid). Martin had become a self-taught expert on low-flying over Germany. He'd been doing it successfully for two years at that time.
Martin's system was very simple. He had learned that if one flew a little lower than the main bombing force, one would avoid the enemy night-fighters, as they would be concentrating their efforts in the main bomber stream. If flew lower still, than the heavy flak would burst above you harmlessly, since the big guns were designed for high-altitude planes. And, if he took it very low -as in, right down to tree top level, he found that he could get past the light flak guns alright because he could be past them before the could draw a bead on him. He had even found away to minimize the risk of barrage balloons. He figured they wouldn't be along roads or canals, so he followed those whenever possible.
Martin and -almost as importantly- his crew had been about the first aircrew Gibson selected. Martin was the an expert at low-level work. His navigator, Jack Leggo, was the top navigator in the squadron and had become highly experienced at navigating at tree-top height. (Which is much different than high altitude, since there one can't see the big landmarks from far off and maps must be of a smaller scale). Plus, the route in and out was carefully plotted to go between the known flak positions. And, his two gunners had been with him for those two years of low-level operations, and had become experts at picking off search lights and light flak batteries.
Low-flying was crucial for the dams operation. Not only for dropping the bomb, but for getting to and from the target. Since the raid took place on the night of a full moon, 617 was the only squadron flying. (The full moon made the threat of fighters too great, so the main bomber force always had the night off at full moon periods; unless some special task was needed). At the normal, high-level height, a lone squadron would be decimated by enemy fighters. (The main bomber force was better off against fighters because they travelled in huge numbers, which split up the fighters.) But, by travelling at low level, they made it much harder for radar to pick them up, and they were hidden from overhead fighters by the dark ground. (An enemy fighter pilot, looking down, wouldn't be able to see a bomber against the dark ground. That was why it was crucial that 617 be safely back out over the North Sea before the sun came up and they could be spotted.)
Of course, low-flying didn't guarantee safety. There was always the chance of a light flak gunner scoring a lucky hit. (The downside of flying so low was they couldn't maneuver much, especially with the bombs on board). As well, searchlights could blind temporarily blind a pilot and illuminate the aircraft, making it vulnerable to flak. And, there was always the risk (especially to navigators not used to low-level operations) of going off course and flying over a heavily defended target where a concentration of flak could be lethal. That is likely where the majority of 617's casualties came from.