So I'm not one of the World War II flairs here by any stretch. But I think the idea of the Normandy landings being a bloodbath for the first wave are heavily influenced by the opening scene in Saving Private Ryan. u/the_howling_cow has an answer here about that scene's accuracy.

However it's important to remember that the hardest fighting was at Omaha Beach and Point du Hoc, with Sword Beach probably close behind that (mostly because it was the one place where the Germans were able to briefly mount a counterattack). Which is not to say that any of the beach landings were bloodless, but that if you watch the theatrical version of Omaha Beach you might come away thinking that people were just getting mown down before they even left the water. Other beaches had relatively light casualties, probably lightest at Utah Beach, where Brigadier General Theodore Roosevelt, Jr. landed with the first wave. He's a great example because while he was under fire and facing danger, he was still a 56 year old general officer/son of a former president walking around with a cane, and survived unscathed (his son Quentin Roosevelt was in the first wave at Omaha and also survived).

Overall we're talking about a total of 156,000 Allied troops participating in the D-Day landings or parachute drops, and 4,414 being killed (out of maybe 10,000 total casualties). I will try to not apply Eastern/Soviet Front bias here: it's not nothing, but it also wasn't really a massacre/bloodbath for the first wave. You can compare it to something like the Battle of Tarawa, which saw a total of 35,000 US troops land against 5,000 Japanese personnel, and about 1,000 Americans killed and over 2,000 wounded in three days to secure a .59 square mile island - and this was considered a shocking bloodbath for American troops coming ashore.

As others have noted, that 90% figure is not accurate at all (despite there being memes saying that circulating on social media). Everyone from the soldiers in the first boats to the generals offshore knew that the landings were going to be dangerous, but actions were taken to ensure their success. Although none of the beaches (Omaha was just one of 5 landing beaches) had what one might term a cakewalk, the other four beaches faced defenses that were far less effective because of an assortment of aerial bombing, naval bombardments, and smoke screens that were planned and delivered in the hours and minutes before the landings. The problem was that the pre-landing bombing that were supposed to suppress the defenders of Omaha beach fell behind the main lines of defense and the naval bombardment was too brief and ineffective. The soldiers landing on Omaha were expecting to face an enemy who had been disrupted and damaged by bombardment and shelling but ended up facing an enemy who had been almost untouched by the pre-landing bombardments.

Once the boats landed, there was no retreating. You couldn't get back in the Higgins boat and go back to the transport ship; the troops could only go forward across a beach that had little to no cover under the unrestrained and unsuppressed fire of well-placed defensive positions. Hence, the higher casualty rates of the first landings on Omaha and the precariousness of their position on the beach after landing.