From the southern entrance to the caves underneath Seafoam (guarded by a Trainer in Generations III and IV), Route 20 curves west, passing south of Route 19's western terminus through a narrow channel before expanding to the width of Route 19 west of the Seafoam Islands. In every Generation, there is a small rest platform just west of the Seafoam Islands (though this has become a single, large island in Generations III and IV). This rest area is occupied by a Trainer in Generation I, III, and IV.
Depending on the generation, the layout of Route 20 between this point and Cinnabar varies. In every generation, however, the essence is constant, as the route continues directly west to Cinnabar, where it encircles the island and meets Route 21 on Cinnabar's north side.
From the rest station, there are three more Trainers, two of which are located very close to each other, before arriving at Cinnabar. There are also two more Trainers along Route 20, one near the western shore of the Seafoam Islands and another just south of the Seafoam entrance on Route 20 in the water.
The length of Route 20 was cut by half in the second generation, which resulted in the number of Trainers being cut to three. Also, the labyrinth of rocks that were present around Seafoam, prohibiting direct access to Route 19 from Route 20 and vice versa, were removed. The only item of interest between the rest station and Cinnabar Island is a lone Trainer southwest of the platform.
Unlike previous generations, Generation III featured no less than four rest islands, two of which contain Trainers. From the rest island present in all generations westward, the route passes north of a small island (with Trainers directly to the north and west of it) before heading south of the largest rest island on the route, north of another small island with a Trainer and north of another Trainer before coming to Cinnabar.
The route is the same in length as Generations I and III. The only noticeable change is that the rest islands from Generation III have combined into a large strip of land that goes along the bottom of the route. There are four Trainers on this island as well as three Swimmers and a Picnicker at the Seafoam entrance. Unlike in Generation II, the barrier that forces the player to travel through the islands' caves remains.
In the first appearance of this route, the rock boundaries follow a strict path. Rocks blocking the way, forcing people to travel through the Seafoam Islands. There are two signposts numbering the islands, one outside each cave. There is also a wooden pier-like island in the west section.
The route is shortened by nearly 30%. The west island doesn't have an entrance, and the cave of the east one is now Blaine's gym. Due to this, the rocks blocking the water path are removed, and there is just one signpost, indicating the Gym and the Gym Leader.
In Generation III, the rock formations are not straight, making the route seem more natural. The Seafoam Islands are remodeled and made bigger. The characteristics of the Seafoam Islands are essentially that of Generation I. The west island is now a tiny sandbar, and three other sandbars are added near the original.
In this generation, the route is very similar to its appearance in Generation III. Unlike Generation II, the path is blocked again and the player is forced to travel through the cave, but the Cinnabar Gym is still in the east cave. Now one signpost indicates the landmark's name, and the other has the Gym information. The four tiny islands in the west are now merged into one large sandbar in the southwest section, with some Trainers on it. Several rocks are now present in the middle of the route.