If you enter a end gateway portal while riding in a boat it will do one of two things:

In both cases, the f3 menu displays that the player has traveled to the correct co-ordinates, but the chunks have not loaded in as it does say "waiting for chunks..."

1) It will take a while to load, then kill you from fall damage (NOTE: not void damage, it says "hit the ground too hard") . If done in creative mode you will die from void damage, which seems to be a weird inconsistency.

OR

2) It will not load and you will fall into the void, but stop falling at y=-65 and keep you in a sort of limbo state where nothing seems to happen. Doing "/setblock ~ ~ ~ cobblestone" will not have any visible effect, but if you do "/tp ~ ~ ~" (this teleports you TO THE POSITION YOU ARE CURRENTLY AT) you will appear back at the end gateway you started at, but the chunks will not load properly by themselves (you will see the outline around the block you are looking at and also some particles from endermen etc.), you have to place/break blocks or use f3+a to reload them manually. One of the bedrock blocks in the end gateway will have changed to cobblestone (from the /setblock command earlier). This second result I have not been able to recreate reliably.

P.S. For anyone whos interested i found this out while working on how to get the How Did We Get Here advancement for 1.13. With the new dolphins grace, slow fall and conduit power effects this already really difficult advancement got even harder. I was trying to move a wither skeleton to an end city.