I've been trying to get under the ice in ZD as an adult, and I can't manage it. Here's what I do:
1. Use the hookshot from Epona (while swordless) on the scarecrow on top of the lake laboratory
2. Get a Guay t follow me
3. Fly to the place where the stream from GV goes into LH, then get the guay to knock me into OOB water
4. Here's what I fail: there should be a way to swim over to underneath LH, bu when I swim towards it along the stream, I always end up falling out of water, even if I swim really close to the in-bound stream.
I've tried doing this on the side of the stream furthest from the laboratory, and it works, but it's nearly impossible to get to the loading point cuz you really can't see where you are... While on other side, I can see where I'm going and hopefully hit the loading point to ZD, but I simply can't get to the main water body.
Also, when using NG+ to get the biggoron's sword on B, if you use the (broken) giant's knife, does it give you that or do you still start with the BG sword? And how do you escape the forest, since TSC doesn't work with BGS? I've tried walking while talking, but I can't seem to manage it. Any tips?...