Just wow. Obtaining it is one thing, but making a video of it AND keeping a straight attitude the entire time?! You are one step above me. If I were you, I would lose my f*cking mind.
I did it pretty much the same way, with only two differences:
1. It's best to rush to the pier as fast as you can, so you have time to steal a Speeder, and position it so that it faces the direction of the Maverick, to minimize the risk of vanishing. Personally, I used a Bickle '76, which I highly recommend.
2. It's not necessary to push this thing completely out of the water. As long as its rotors are constantly spinning AND you can enter it WITHOUT drowning, you're good to go.
NOTE FOR PSP USERS: I told Lennart - via PM, now I'm saying it in public. As mentioned a few times before, I'm noticing that vehicles I take out of the garage don't vanish, unless I destroy them, or save and reload. If this is the case for you, you can leave a Heavy/WP/DP bike (preferably the Freeway or Angel, since they are the heaviest, and can be obtained ANYTIME, in case something goes wrong) in Harwood, to GREATLY aid you with the final Trial. Personally, I didn't think of this at the time I obtained the helicopter, so I did it the hard way, just like zazaza691. If your precious bike stays there, then the difficulty will be cut down by a third, and it should be possible to make it in one go, assuming "Taxi Driver" doesn't crash.