“Sterngating” (stern + tailgating) is when a tailgate picnic is held from a boat instead of a car, van, or pickup truck. The tradition appears to have started at the University of Washington and is also practiced at the University of Tennessee.
 
The New York (NY) Times wrote about Washington’s tradition on October 13, 2011:
 
“Once docked or anchored, they tailgate with a twist, a practice the locals have alternately called boatgating, sailgating and sterngating.”
 
The word “sterngating” has been cited in print since 1994 and 2000; “sailgating” has been cited in print since at least 1991 and “boatgating” has been cited in print since 1996.  Sterngating differs from tailgating in that sterngating usually has less grilling of foods (to prevent boat fires).
 
   
Google Groups: rec.sport.football.college
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From: .(JavaScript must be enabled to view this email address) (Jason Whitt)
Date: 1 Aug 1994 02:52:14 GMT
Local: Sun, Jul 31 1994 9:52 pm
Subject: Re: Tailgating
 
In article <1994Aug1.024457.2…@afit.af.mil>,
Brent A Peacock

wrote:
:
:And you can’t beat tailgating in a boat (piergating?).
 
I think this would be “sterngating”.  (Or bowgating, I get em mixed up.)
I hear they do this at the Naval Academy as well.
     
16 September 2000, Fort Worth (TX) Star-Telegram, “Tailgating takes on nautical twist at Vols games,” Sports, pg. 10:
The Volunteer Navy weighs anchor and it’s one big “sterngating” party.
 
NBC Sports
Tailgating Top 10
Associated Press
updated 2:09 p.m. ET Aug. 20, 2004
(...)
Tennessee
The “Volunteer Navy” is comprised of hundreds of ships that dock on the nearby Tennessee River on game days, doing their partying on the water.
 
Washington
An even more expansive body of water, Lake Washington, is not far from Husky Stadium. Plenty of boats for “sterngating” and a seaplane drops in from time to time, too.
 
Google Books
The Ultimate Tailgater’s PAC-10 Handbook
By Stephen Linn
Guilford, CT: Globe Pequot Press
2007
Pg. 138:
Sterngating
You’re not going to find a tailgate lot like Lake Washington anywhere else in the country. I don’t mean along Lake Washington; I mean on Lake Washington. Thousands of fans park their boats on the lake to “sterngate” before home games.
   
The Florida Times-Union (Jacksonville, FL)
Saturday, September 29, 2007
Sterngating the River City
(...)
Nick Simonis calls tailgating on the boats “sterngating” or the act of tailgating with water below you rather than asphalt.
 
So is the food the same when you’re sterngating? Well there are some differences as open flames are not allowed out on the docks. Most of the yachts had full spreads set out for their guests. Instead of just one table of hamburgers, hotdogs and some steaks you find platters, catering style meals and more.
 
New York (NY) Times
Tailgating Crowd’s Unlikely Roar: Ahoy!
By GREG BISHOP
Published: October 13, 2011
SEATTLE — The University of Washington’s football stadium loomed ahead, beyond the traffic, as the Miles family steered toward its usual parking spot one Saturday last month. For 50 years, the family members have arrived at Huskies home games through this entrance the same way: grill gassed, coolers stocked with microbrews, clad entirely in purple.
 
Their routine would seem like a typical tailgate, if only there were cars.
(...)
Once docked or anchored, they tailgate with a twist, a practice the locals have alternately called boatgating, sailgating and sterngating.
(...)
Husky Stadium opened in 1920, and soon after, the boat tradition started, with fans stashing vessels in tall grass not 200 yards from the end zone. Docks were built around 1960, according to Dave Torrell, the curator of the university’s hall of fame, and early transportation from anchored boats often came from members of the rowing teams in exchange for tips.
 
PRWeb
One of the Most Unique Ways to Tailgate Revealed: Boatgating
Discover Boating names best stadiums for the latest trend in on-water tailgating.
Chicago, IL (PRWEB) August 23, 2012
(...)
Boatgating, also dubbed sailgating, stern-gating or transom-gating, has become a unique way for boaters and sports fans to experience the tradition of tailgating with an extra element of fun. Game day starts and ends with a relaxing cruise on the water, much more enjoyable than sitting in pre- or post-game traffic. For those with boats furnished with access to a grill, refrigerator and ample seating area, boatgating becomes even more convenient. No lugging of grills, chairs or other cooking gear when it’s all on board.