“I’m on a seafood diet. Whenever I see food I eat it.” Brooklyn/Los Angeles Dodgers player and manager Tommy Lasorda has used this quip—a play on the words “seafood” and “see food”—since at least 1977.
 
The joke has been in print since at least 1970; Lasorda probably popularized it, but didn’t coin it.
   
   
Wikipedia: Tommy Lasorda
Thomas Charles Lasorda (born September 22, 1927 in Norristown, Pennsylvania) is a former Major League baseball pitcher and manager. In 2008 he marked his 59th year in one capacity or another with the Brooklyn/Los Angeles Dodgers organization, the longest tenure anyone has had with the team (Vin Scully joined a year later).
   
31 December 1970, Aiken (SC) Standard, “Here’s Rufus” by Rufus W. Gosnell, pg. 4, col. 7:
HAVE YOU heard about the new seafood diet? You see food but you don’t touch it. 
 
17 February 1971, Long Beach (CA) Press-Telegram, “Travel Tips For Oldsters” by Pat McDonnell, pg. B6, col. 1:
“I’ve got to lose 15 pounds to be in shape for traveling and my wife has me on a seafood diet—I can see food, but I can’t eat it.”
 
Google Books
Audience
By Geoffrey Ward
Published by , 1972
Item notes: v.2 no.5 1972 Sep-Oct
Pg. 53:
He turns again to Neil who tells a joke about a seafood diet he’s on. “If I see food, I eat it”
 
9 December 1975, Uniontown (PA) Evening Standard, “Jokes” by Anthony Bartoni, pg. 26, col. 5:
Denise: I am on a seafood diet.
Tony: What kind is that?
Denise: When I see food, I eat.
 
22 October 1977, El Paso (TX) Herald-Post, pg. A6, col. 2:
Tommy Lasorda, the Dodger manager, doesn’t look like he misses many meals…He said he was on a seafood diet. “When I see food I eat,” he explains.
   
Google Books
February 1978, Baseball Digest, pg. 40:
There’s no stopping or topping Lasorda as a standup comic. here’s how he describes third baseman Ron Cey: “He’s so little, he can play handball against the curb.” On himself, Lasorda says, “I’m on a seafood diet. Whenever I see food, I eat it.”
 
Google Books
Behavior Analysis and Behavior Modification: An Introduction
By Richard W. Malott, Mary Tillema and Sigrid S. Glenn
Published by Behaviordelia
1978
Pg. 131:
“Hey, what kind of diet did you say you were on?” he demands. “A seafood diet,” you answer. “I see food and then eat it.”
 
8 May 1987, Los Angeles (CA) Times, sports, section 3, pg. 4:
His diet sounds like Tom Lasorda’s seafood diet: “I see food and I eat it.”
   
2 March 1989, Sacramento (CA) Bee, pg. C2:
Among Mary Aldridge’s favorite slogans is one she found on an apron in a gift shop: ““I’m on a seafood diet—I see food and I eat it.’‘