Intentional Purpose for Desmond receiving the boat
It is possible that Libby placed the tracking device on the boat with the hope of Desmond (unknowingly) leading her to The Island. However, since it took her over three years to find it, the tracking signal (due to the electromagnetism of The Island) couldn't have been very successful.
Someone else may have used Libby as a means of getting a boat to Desmond. If Ms. Hawking was right and Desmond needs to push that button to save the world then it was vitally important that he get a boat for that race.
There may not actually be a David, it could all be a story made up to prevent Desmond being suspicious.
Desmond sails it back to civilization
This boat is Desmond's most likely route back to civilization, following the series finale. A short hike by him (with or without Ben and Hugo) would take him to the dock where Locke's outrigger is, and then to Hydra Island and the sailboat.
It is Libby's Constant
Libby somehow knew that Desmond would crash on the Island with Elizabeth, possibly from Jacob, Widmore, Dan Faraday or even Desmond himself and she also knew own day she would also crash on the Island, possibly from one of the above. So therefore she gave the boat to Desmond knowing that the Elizabeth would be her constant if she had made it to the time traveling.