The bug
When attempting to walk on boats on water movement becomes jittery and slowed as if the player is getting stuck on the boat. The same applies to some extend to the moving top of a shulker.
Also affects items dropped on the boat.
Related issues
clones
is duplicated by
relates to
Attachments
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Duplicate of MC-156979

Duplicate of MC-136367

Although the effects of MC-136367 can lead to MC-156980, players can simply jump onto a boat when one is in water at any time and still experience this issue, without the need to mount and dismount it (although that makes reproducing this easier).
Also, repeatedly jumping on a boat can buildup fall damage due to the bug.

Confirmed for 19w34a.

Confirmed for 19w35a.

Confirmed for 19w36a.

Confirmed for 19w37a.

Confirmed for 19w40a.

Confirmed for 19w41a.

Confirmed for 19w42a.

Confirmed for 19w44a.

I really hope this bug gets removed until 1.15 release, it would be awesome to see a "fully working" release on the first try. Imagine the clean transition from 1.15 to 1.16 without any other release in between.
Agreed, it would be nice if all bugs could be fixed until Tuesday. Unfortunately there's only a finite amount of time until then.

At least you don't get killed by jumping out of the boat any more. Unfortunately this bug wasn't a part of that one...

The stuttering effect only applies if the boat is not sinking. When spawning a boat and walking on it immediately, this bug doesn't trigger.
Relates to MCPE-59679.

Confirmed still an issue in 1.16 pre7, note: This video was recorded in singleplayer.
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Still an issue on 1.16.1. I'd love to see a fix ^^
✔ Updated stuttering with boats: only happens when they are on water (https://discordapp.com/channels/647810384031645728/648555618994618378/754172816802316368)
Can confirm in 21w03a.
Can confirm in 21w06a.
Can confirm in 21w11a.

Can confirm in 1.17.
Can confirm in 1.17.1.
Can confirm in 1.19. This issue now also can be seen with chest boats.

Can confirm for 1.19.4.

Could we please remove the "on water" part of the title? There are multiple duplicates which say this happens even on land

Is there a way to reliably reproduce this? I'm having trouble with this.
I've noticed it's much more reliable to reproduce if you enter and exit the boat immediately after placing it down.

I can reproduce in 1.20.4 and 23w51b.
This seems to be a client-server desync.
Here are consistent steps to reproduce:
Place a repeating command block with the following code in it:
tp @e[type=minecraft:boat,sort=nearest,limit=1] ~ ~.6 ~
Place a boat close to the command block.
Power the command block.
While the command block is powered, sit in the boat and get off.
You can now un-power the command block.
❌ Attempting to stand on the boat now causes stuttering movement. After a while, the client's boat position might be synced to the correct place.
In this example, you can notice that even while the command block is teleporting the boat every tick, you can row it away from the command block and it won't teleport back when you exit the boat.

Might relate to MC-247418; This seems to be due to the OnGround Player NBT not being set properly when clipped very slightly into a boat or shulker (probably due to floating point error), causing rubberbanding.

Affects 1.21 Pre-Release 4

Can confirm for leashed boats that are floating midair in 1.21-rc1:
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Can confirm for 25w16a. Still in the game