The Swiftskin boots can be aquired by completing the Pier. After you entered the boat, sail to the left (towards the waterfall) the hidden cave is somewhat in the middle of the waterfall. Inside just keep sailing upwards untill you come across a bridge. You will find 2 treasure chests. one containing money (around 2k i think) the other containing the Swiftskin boots.

Swiftskin boots
Sailing left
Sailing upwards