System changes for a seafaring campaign

[Gaekub]

I have an idea for a campaign that takes place in an island world with highly dangerous seas. However, I want ships to remain the only (or at least primary) way to travel between islands more than a couple miles apart even if PCs and NPCs have access to higher level magic.

Obviously things like Teleport need to be removed, but what else should I change? I could just remove 9th level classes and trust that by the time 6th level casters get access to similar effects they have better things to do, but I don't know if I want to remove that many classes.

Any help would be appreciated.

[Rylar]

You could always just change teleport spells to have interference when they cross water. Same with things like flight. After that just tell your players your expectations and that you will adjust any workarounds they come up with.

Are you going to let them swim between islands? If not may want to take a look at shape changing abilities.

[Gaekub]

Rylar wrote:

You could always just change teleport spells to have interference when they cross water. Same with things like flight. After that just tell your players your expectations and that you will adjust any workarounds they come up with.

Are you going to let them swim between islands? If not may want to take a look at shape changing abilities.

That's not a bad idea. I hadn't even considered fly.

My main goal is to make the oceans still feel like an obstacle, even at higher levels. I like the idea of large bodies of water interfering with certain types of magic, so I may think about that.

If they want to swim, I'm fine with that, even if they all somehow manage to turn into megaladons or something. They're still interacting with the sea and what lives within it.

[SanKeshun]

Have teleport spells function normally, but they actually just move you along the ground super fast. The magic helps you avoid or move through things like water, trees, buildings, and you move over mountains and islands normally, but the spell can still be interrupted by substantial obstacles, including any form of non-plant life.
If they teleport across the ocean, have each player roll d%. On a roll of, say 15 or lower, they hit something that interrupted the spell. Everyone else crosses over normally, with no sign anything went wrong, but the affected character is now at the bottom of the ocean, and guaranteed next to an enemy. They might still risk it, but it's a huge risk.
For overland flight and all, just don't let them go that high above the ocean surface. Sharks and things can jump.