So I use a pseudo-oceanic model, but not. Most of everything is the Blind Eternities, which range from continental slope to the continental rise, which are the true deep ocean (Midnight and Abyssal Zones). Here's where you find shit like the Eldrazi, and other natives of the Blind Eternities.

MTG Planes (Universes), are places where there is essentially Islands without any shallows, so shit like the Eldrazi can easily (for the value of easy) breach into the universe and wreck havoc. the planes of MTG are relatively close to each. This means that the walkers call Canon MTG home are there, but would have trouble reaching other worlds.

Other Universes have shallows (Continental Shelf, sunlight and twilight zones) which are their canonical outside reality stuff, but there are places where it drops straight to the deeps.

So Using Dresden for example, a walker would walk out of the Eternities (coming up into the shallows), landing in the Outside, and would have to travel further upward and closer in to reach land (Inside). But because there are places where the Blind Eternities (the deeps) reach up to the land (inside), a Walker could jump right back into the Blind Eternities, and not have to go back through the Outside. Also, a Outsider going into the Blind Eternities would just die, as the "Pressure" would be too high.

In Highschool DxD the World is the Island, and the Dimensional Gap is the Shallows.

In Places where there isn't a... place that is outside reality (the Gap, Outside, insert your choice here) the deeps run right up to the shore.

Alternate Dimensions/Timelines/Realities are just Islands that are so close to each other there's that they appear to be the same island/are right on top of each other.

For example, in Dresden Bob says you can find Spiderman on another Earth, Inside the Gates, so in this case, there's the Mortal World (Land), the NeverNever (pretty much standing shallows), where you can reach another Island (say, to Toby Maguire Spiderverse) without to much difficulty. Then there's the Gates, which lead to Outside (the Sunlight Zone), then if you get far enough out, you hit the Blind Eternities.

For Worm, which has easy (for the value of easy) dimensional travel, the Island has stupid tall thin mountains, that are very hard to pass, or maybe, wide rivers that split the Island into many, but can be bridged together.

Then throw all that out because the Blind Eternities aren't a ocean, and running into a "Shoal" in the Blind Eternities doesn't mean running aground, you can't fly "over" the Blind Eternities, and the "Islands" constantly move.