Betancourt hit a two-run double in second and connected for his three-run homer in the fourth off Giants starter Matt Cain.
Knuckleballer R.A. Dickey (1-0) made a spot start for the split squad Mariners and held the Giants scoreless for five innings. He gave up just one hit while walking three and striking out two.
"I showed enough speed that I was effectively wild," said Dickey, who's trying to make the team as a long reliever. "I was able to keep their timing off. The pitch was really moving tonight."
Mariners relievers Brandon Morrow, Ryan Rowand-Smith and Sean Green finished out the game for Seattle, giving up two runs and four hits.
In the top of the eighth, a flock of seagulls almost completely surrounded Giant center fielder Fred Lewis as he stood in the field. The gulls had descended on AT&T Park in the fifth inning in search of food.
Cain (2-3) gave up six runs -- five earned -- in four innings while striking out six with two walks in his final spring start. He's expected to start the Giants' second game of the season against the Los Angeles Dodgers on Tuesday.
The Giants (8-21) have now lost five straight and have worst record in exhibition baseball.
