I have some ideas for ways to upgrade the Boat, in a similar vein to the Chest Boat
The submersible is a simple upgrade that is crafted from two boats and wax. It has the appearance of two boats stuck together by their rims, with a pair of oars sticking out somewhere in the lower part. This boat only accepts one entity, but it has the advantage of being able to dive into bodies of water and move about much quicker than swimming. It unfortunately doesn't provide any extra air
The ender boat is crafted like a boat, but with purpur blocks in place of wood, an eye of ender in the center, and 3 shulker shells in the remaining row. This recipe reflects its appearance as a purple boat covered by a shulker-shell, with a seat and base like an eye of ender. This boat is designed to be used in the end, and as such has the ability to slowly float along when it is directly over the void. This allows the player to go between end islands easily
The railboat is a simple and potentially silly idea: It is crafted with a minecart and a boat. This combination, obviously enough, can work as both a minecart or a boat
The submersible could work. The ender boat (which I'm imagining as a boat that doesn't fall, just glides along air like water but slower) I'm not sure about. The railboat does sound hilarious, and while a single vehicle capable of behaving as both a minecart and boat might not work, perhaps a way to automatically roll on/off of rails and water would be workable.
All three of those sounds fairly okay, but the submersible is the only one I could fully support. I think the point of The End's void is for the player to use their brains and place enough blocks to get around, instead of freely gliding everywhere. I don't know if I can support that one, but I wouldn't complain if it was added. The railboat is not over the top enough for me to dislike it, so... I guess it's fine.