According to [1], [2], and [4], multiple beaches in Massachusetts were closed this weekend by the Massachusetts Department of Public Health because of unsafe swimming water. The [1] article specifically mentions 35 beaches being closed, while [2] notes that two South Shore beaches are closed due to bacteria levels. [4] also reports on multiple beach closures but does not specify a total number. [5], however, lists over 40 beaches as closed in its headline and body text.
The reasons for the closures are consistent across all sources: unsafe swimming water conditions have led to beach closures. The nature of the issue is described similarly by [1] and [2]: "unsafe swimming water" which can result in illnesses with symptoms such as gastrointestinal, respiratory, or flu-like issues. [3] provides more context by mentioning that this situation occurred during a heat advisory affecting most of the state.