Police say the small dog jumped into Hampton Ponds in Westfield at about 6:30 p.m. on Monday, prompting 64-year-old Donald Cyr to follow his pet into the lake.
Police tell The Westfield News that he became distressed and his wife, 61-year-old Patricia Cyr, tried to rescue him.
Other boaters, alerted by screams, helped pull the couple from the water. Efforts by rescue personnel to revive them failed and both were pronounced dead at hospitals.
The dog survived and was taken to the municipal animal shelter.
The deaths remain under investigation.