Before anything, I did NOT ask you if it was possible to optimize or not... I told you to DO it. You can't figure the game physics out without playing and feeling them yourself.
About the underwater stages now.
The faster speed: do a spin, wait 24 frames (or wait 2 frames before it returns to its swimming stance), boost, wait 10 frames and spin again... repeat while possible.
You can boost, wait 11 frames, boost again... repeat -- but that's a little slower.
There's the crate problem too... rows of crates laying on the ground don't affect your speed, but stacked crates reduce the speed if you do a horizontal spin. And don't forget that if you destroy the lower crates in a pile, any crates that fall upon you will be automatically destroyed too.
Just in the case you're feeling lazy again... watch
this.
I'm looking at your new run right now, but I think you should not advance too fast. You should concentrate your attention on the small things.