When riding a boat, the cape will start twitching very rapidly (perhaps every frame) and move very glitchedly. It's easy to achieve at high speeds like on blue ice, but I've managed to do it on normal water.

Tested on Fabric/1.17.1 and Fabric/1.18-pre5 on commit 5cacfd4.

Perhaps I should have been clearer, side-by-side comparison on YouTube.

The vanilla cape movement is weird because it's clamped to its own detriment, but it's far from rapid and glitchy.

Ah, weird. I was trying to reproduce this on a straight ice road (since it's easy to WE and I had no other detail)

Only seems to happen if the ice road is long enough, so I suspect this is related to vanilla capping the cape delta and snapping it to the player position with no interpolation when it exceeds a certain distance.

I guess I could "fix" this by not relying on the vanilla cape code and implementing it all in Common; the NSSS port already contains a reimplementation of the cape position system...

Yep. Reproduced with a vanilla cape.

Are you lerping the movement by any chance? One of my mods, CapeTweaks, has an option to apply lerp on the cape angles and when I went boating with it turned on, the Vanilla cape went bonkers, much like in the video.

Ears does lerp movement; I suppose that makes sense.