In the stage play Murder on the Nile, the Lotus is a paddle steamer cruising down the Nile. Like the Karnak in Death on the Nile, Christie most likely based the boat on the PS Sudan, a paddle steamer which she travelled on in 1937 on a cruise up the Nile.
According to the stage directions, all the action in all 3 acts take place in the observation saloon of the Lotus between Shellal and Wadi Halfa. In Act One the boat is at Shellal where the passengers embark. In Acts 2 and 3 it is at anchor near the Temple of Abu Simbel.
As all the action in the play takes place inside the observation saloon, this is the only part of the Lotus which the audience will ever see. This scene from a 2015 production of Murder on the Nile at the East Lane Theatre in London shows a typical set of the observation saloon.
Agatha Christie most probably based her play on a cruise on board the PS Sudan in 1937. This boat is still in service offering Nile cruises. It is not clear what the floorplan of the boat was in Christie's time. Today, the top deck, under the white awning is the sundeck and the solarium but obviously Christie intended the action in her play to happen in an enclosed space. Below the top deck, most of the upper deck is used for cabins. On the lower deck, turning right as you on board via the gangway, is the dining room/lounge/bar. We can assume this is the "observation saloon" of the boat.