That allowed the sprinters to race for the line while the main men stayed safe in the pack and out of the way. Roglic (Team Jumbo-Visma) leads Britain's Simon Yates by an unchanged 35-second margin. The Slovenian was seen talking to the race director about the conditions with 70km to go.
Colombian Fernando Gaviria, for UAE Team Emirates, finished the stage second behind the Bora-Hansgrohe rider with Frenchman Arnaud Demare third for Groupama-FDJ in a stage that was also neutralised for the first 2.7km. Ackermann, making his grand tour debut, also won Sunday's second stage from Bologna to Fucecchio.
"All the stage was scary, all the descents, you cannot see that much in the peloton because of all the water," said Ackermann, wearing the cyclamen jersey for the sprinter classification leader. "It just was lucky that nobody crashed."
The German said it had been effectively a double sprint to the finish for him after he had to brake in the last 250 metres before harnessing all the power in his legs in a final dash for the line to take Gaviria by half a wheel. The sprint finish, in the seaside resort south of Rome, was all the more impressive for the pools of standing water on the asphalt.
Dumoulin, feeling the effects of a big crash on Tuesday, had started the stage but the Dutchman abandoned shortly after the start with his focus now on the Tour de France. "I wanted to finish it. I am not ready to go home yet," the 2017 champion told Eurosport.
"I didn't want to be at home in two days' time and my knee would be less swollen and I would be able to do a ride and feeling sad that I didn't try and maybe if I'd pushed through and with some painkillers today it could have been possible. "I would have always asked myself that question and now I ask myself and know the answer."
Belgian rider Louis Vervake was caught 23km from the finish after staging a long breakaway. Thursday's sixth stage is 238km from Cassino to San Giovanni Rotondo, with a fast opening 190km and no major climbs before ascending through hairpins. (Reporting by Alan Baldwin in London, editing by Toby Davis and Ed Osmond)
(This story has not been edited by Devdiscourse staff and is auto-generated from a syndicated feed.)