OoT - How the Lakeside Lab dive works (and hookshot trick)

There's a flag in the lake scientist actor (flags & 8, which is a short at instance+0x26E) which gets set when you have irons equipped, and gets unset if you are not in the underwater state (which is player->stateFlags2 & PLAYER_STATE2_10). Let's call this scientist flag the "using iron boots" flag.

If you are more than 355 units from the water surface (which is within 5 units from the bottom), and the "using iron boots" flag is not set, then it sets a different flag which lets you get the heart piece (flags |= 4;)

The reason you can't just unequip irons while on or near the bottom of the lab to get the heart piece is because you are in the underwater state, and thus the "using iron boots" flag does not get unset.

The reason hookshotting the crate works is that it unsets the underwater state, which causes the scientist to unset that "using iron boots" flag.

Another interesting thing I noticed while testing this. When you have iron boots equipped, and are NOT in the underwater state, the "using iron boots" flag alternates between set and unset every frame. Because you can hookshot something underwater to unset the underwater state, after hookshotting the crate, the "using iron boots" flag will be alternating every frame while you are standing on the bottom of the lab water. Then, if you unequip the irons on one of the frames it is unset, the game will set the flag allowing you to get the heart piece, as delayed as you want. Unequipping on the other frames does not work.
In the video below, you can see it alternating between 9 and 1 (it is really switching back and forth much faster than the video shows, that is just the memory viewing software I was using making it look like it is switching every few frames), which is the "using iron boots" flag alternately getting set and unset every frame. I unequip irons on one of the frames where the "using irons" flag is unset, and can now get the heart piece.

https://www.youtube.com/watch?v=ReDGPnjbk_4