In order to get to the purple coin in the harbor I used: A speed buff ( In my case it was base monk run speed and a pendant of time), House P jump buff, and a pair of rented skates. Do not equip a feather fall item the ramps are too steep and you will not fall steeply enough to stay on them.
While abundant step helped me get my first 10 white coins I did not need it or use it to get to the purple coin.
Start by heading up to the top of the ramp near the harbormasters house. You need to start at the right hand side and angle to go off the bottom left corner of the ramp.
Once you are sliding down the ramp you will need to hold forward to gain speed. Just before you leave the ramp jump.
The first landing is the most forgiving.
As long as you land on some portion of the downward ramp you should be fine.
The farther back you land the sooner you will need to jump to keep from over shooting the next landing.
You may need to hold your back key to avoid missing the landing for this next part. The goal is to end up in the far right corner. This will give you time to line up your next jump.
Once you are there use the number pad to move the camera without turning your character. Once you line up your path use mouse look to turn your character.
Let yourself slide without using the forward key till you start down the ramp.
You will need to jump just before the end of the ramp.
For the last ramp it is very important that you land just past the back edge of the ramp and jump just before you leave the ramp otherwise you will not get enough height to reach the coin. If you hit the very back edge you will land on it and it will slow your fall.
If you are the only one that is attempting this the coin can take a while to show up again, however if someone other than the last person to grab it gets close it will respawn for them. I was once right behind someone else and I was able to get the coin even though they grabbed it right before I got there.