I'm learning how to do any% as my first Ocarina of Time speedrun. Right now I'm learning how to do GIM in Deku Tree. When I try to do dive cancle I hear both the C-Up sound and Link diving. When I cancle C-Up to start swimming to the vines, Link suddenly surfaces from the vines. I've tried pause buffering to make sure I press A and C-Up on the same frame and this still happens. Anyone know what I'm doing wrong?

I'm not entirely sure what you mean (but i also play on n64 and am not very good at gim ) but the forums here on src are not that active, all the activity is in the discord server which is linked on the left of the screen. You'll get a faster more accurate response if you ask in the any% channel