“Beer goes down easy on a boat” was an American expression used by the informant’s father. The informant’s father would say this phrase while, obviously, on a boat and drinking (a lot) of beer. The informant believes this expression was simply a way for her father to justify drinking, which she thought was funny. She (the informant) pointed out that he wanted to drink beer anyway, and the phrase just helped him justify it.
The informant also said the phrase has since become a kind of family joke–“beer goes down easy on a boat” is used as a joke whenever family members are drinking beers together–such as saying “beer goes down easy sitting at the kitchen table” while sitting at the table drinking beers.
I agree with the informant’s analysis. I think that being on boats in general lends itself to drinking, and this phrase was a telling and hilarious justification of one too many beers.