The Headhunters — Peter Lovesey

Gemma loathes her sleazy boss; Jo is her confidante. On a double date with Rick and Jake, they discuss forming a mutual murder society, in jest of course. The next day, Jo, walking on Selsey Beach, discovers the corpse of a drowned woman, a stranger. But one of the men in the lineup at the police station is Gemma's date Jake, who Jo rather fancies.

Then Gemma and Jo discover the corpse of Fiona, Gemma's annoying colleague from work. And Gemma's boss is missing. When an older woman whom Rick was involved with is drowned in her pool, the police begin to close in.

Can this outbreak of deaths by drowning be coincidental? Or has the joke gone too far?