Constellation Brands Inc.’s big bet on cannabis has been a drag on profits, but the brewer is preparing its next gamble: jumping into the crowded, fast-growing market for alcoholic seltzers with a new Corona-branded drink.

The Victor, N.Y., company posted a $525.2 million second-quarter loss, weighed down by paper losses on its investment in Canadian marijuana grower Canopy Growth Corp. Constellation was one of the first brewers to invest in cannabis and put roughly $4 billion into Canopy, giving it a 38% stake.

Constellation’s...