I don't have a lot of info but a friend of mine told me that there is a HUD (or was) that pings GPS points on the map. And you start this race in a car, then get in a boat, then get in a car again.... and at the end, there's some cool party. There is NO amount of googling that is helping me find this so I'm reaching out to the SL hivemind. Help.

That sounds like the weekly grid drive put on by the drivers of SL group.

Their headquarters is here: http://maps.secondlife.com/secondlife/Bruissac/109/72/36

Or maybe I could be totally off base. I know I did something that sounds exactly like what was described in the OP once with a friend over a year ago.