In my original campaign the first two characters had to book passage on a small skiff to get to a tiny, forest-covered island where they ran into a clockwork horror. To give them more leeway, I gave Josh's character an astral vessel and a crew. This allowed them to go where they needed to, while instilling the impression that everyone would need one.
This does not need to be the case.
Inside the demiplanes (or pockets, dominions, sanctuaries...still thinking of a name for these) where the natural world (or Prime Material Plane, if you prefer) is the most dominant, physics are somewhat more of a thing. So if there is lots of water, you can row to get about. Since there is wind in the astral, there is wind in the demiplanes as well, which can be used to sail.
Outside there is nothing to really slow you down, and creatures can also travel via willing it (or just fly or swim if they could do that normally). So, I see now reason why mundane vessels cannot simply pick up speed before leaving a demiplane, a crew cannot just communally will their ship (perhaps the act of rowing with the belief that it should move the ship, does), or simply have swimming creatures tow the ship.
The benefit of this is that it allows characters who do not have to have access to an expensive magical vehicle to still get around, as well as something to look forward to (ie, an expensive magical vehicle that they can use to get around). Astral vessels still have use (probably higher speed and maneuverability), but to add some variety Josh and I have come up with some other options.
The denizens of Asmodeus, in keeping with their fiendish nature, delight in using black ships that are powered by the souls of the dead. An anima reactor emits an agonizing wail while in use, which can instill a sense of dread in mortals (especially in animals). Even worse, souls contained within a reactor's soulcage can be harnessed in an emergency to fuel magic, in particular necromancy.
Dwarves and humans have managed to invent a kind of magic device that, through the forced collision of incompatible elemental...well, elements, can easily hurtle a ship through the astral. They are fast and environmentally healthy, except when they explode: the insurgence of conflicting elements must be carefully regulated by an engineer to ensure safe operation.
The astral sea is vast, and despite the planar collapse it is still pretty empty. To help deal with the issues of navigation, many sailors rely on the stars or ley lines. Ley lines criss-crossed the world prior to the Sundering, and their energies register with those that can sense spirits. Sometimes they extend from one demiplane to another, but most commonly they are used to orient a sailor.
Not all travel requires the use of a ship. Angel gates are magical constructs, similar in function to a teleportation circle, but rely on the Angel Roads that existed before the Sundering. Activating them requires a substantial amount of celestial essence, prayer, angel hearts, or powerful miracles.
Other forms of travel exist, such as astral whales and dragons (especially in the case of the githyanki), but hopefully this starting list is more inline with the fantastical nature of the setting that I initially conveyed. I will try to do an update of more stuff at least once a week as Josh and I hang out and kick ideas back and forth.
Next week, I think we will talk about a city where the dead go to celebrate before finally passing on.