Mr. Foster entered the water but got caught in the current along with four other swimmers, which included his sons. “This loss is unimaginable, and our grief is immense,” Ms. Foster wrote on a website dedicated to Mr. Foster. Mr. Foster was unconscious and not breathing when he was brought back onto land and loaded into an awaiting Tri-Town ambulance at 4:23 p.m., officials said. Cambridge state Rep. Marjorie Decker is close to the Foster family and grew up with Ms. Foster. Lifeguards are not stationed at the Trustees beach, though there is rescue equipment there for rangers.