I’m an amateur U-boat historian currently writing a nonfiction book on the topic (I actually received my third publishing offer this week.) You ask some great questions, and it's worth noting that Das Boot is both a fantastic film and a very accurate portrayal of life aboard a U-boat. Note that my answers below apply to the Type VII and Type IX model boats, the workhorses of the WWII U-boat fleet. The fictionalized version of the U-96 portrayed in Das Boot was a Type VIIC.

The odor of diesel was indeed ever-present in the petty officers' quarters, which were located in the aft section closer to the diesels (the senior POs slept in the forward section.) The entire boat, however, reeked of diesel...among other things. A tiny kitchenette (which was across from one of the two toilets) separated the PO berthing from the diesel room, consequently, even the food tasted like diesel. This smell mixed with others to create a disgusting potpourri of stink. U-boat officer Herbert Werner described it vividly: “The stench of 51 sweating seamen, diesel oil, rotting food, and moldy bread mingled with the noisome odors that emanated from the galley and the two tiny washrooms. The overbearing smells and the never-ending rocking made the men in the narrow drum dizzy and numb.” The diesel engines were also extremely loud; one minor Das Boot inaccuracy is machinists conversing in the diesel room. In reality, it was impossible to converse there when the engines were running.

Das Boot’s depth charge scenes are also very realistic, to include how deadly (or not) these weapons were. Depth charges could and did destroy many U-boats, but they were actually rather crude weapons that had to detonate fairly close to inflict fatal damage. Postwar analysis showed that any given depth charge attack in the Atlantic theater had about a 6% chance of killing a U-boat. The US Navy’s primary depth charge in 1942, the Mark 7, had a 600-lb charge but still needed to detonate within 30 feet for a 90%+ chance of breaching a U-boat's pressure hull. The same sized charge could damage a U-boat within about 115 feet, however, which was often enough to force it to the surface. Their limitations meant that Allied forces tended to saturate an area with dozens or even hundreds of depth charges. They often damaged morale worse than the boat, but their psychological effect was considerable. Some compared enduring a depth charge barrage to being inside a metal drum while being while it was being smashed with sledgehammers. U-333 commander Peter Cremer described a 7-hour bombardment near Florida: “There were trickles [of water] everywhere. The minutes became eternity, all sense of time was lost."

U-boats could directly communicate with other Kriegsmarine vessels via radio, however, communication was almost always routed through BdU (naval headquarters.) They did typically overheard other U-boats’ routine status report transmissions to BdU, which helped commanders maintain broader situational awareness. When two U-boats needed to rendezvous, such as for resupply or transferring a wounded sailor, BdU would act as the communication middleman and guide the two boats into the same area. The boats’ lookouts would then spot each other and use the signal lamps once within visual range.

Please let me know if you have any other U-boat related questions!

Sources:

UK Admiralty, “Interrogation of U-boat Survivors CB 04051 (103)”; Jak Mallmann Showell, Hitler's Attack U-boats; Herbert Werner, Iron Coffins; Richard Snow, A Measureless Peril; Timothy Mulligan, Neither Sharks Nor Wolves; Randall Peffer, Where Divers Dare; Giese & Wise, Shooting the War; Peter Cremer, U-boat Commander; NavWeaps.com, “ASW Weapons”; AlternateWars.com, “Calculating Depth Charge Lethality”; Pacific War Online Encyclopedia, "Depth Charges"