A murderous marriage / Alyssa Maxwell.

Enhanced descriptions from Syndetics:

Lady Phoebe Renshaw and her lady's maid, Eva Huntford, are preparing for a wedding, but it may not be the happy occasion everyone hopes for . . .

Since the Great War, some family fortunes have suffered, including those of the Renshaws. Despite being the granddaughter of an earl, Julia Renshaw is under pressure to marry for money--and has settled for Gilbert Townsend, a viscount and a wealthy industrialist. He is decades older than Julia, and it's clear to her sister Phoebe--and to Eva, who has been like a surrogate mother to the girls--that this is not a love match. Nevertheless, the wedding takes place--and in a hurry.

At the reception aboard the groom's yacht, there appears to be tension between Gil and several guests: his best man, a fellow veteran of the Boer War; his grouchy spinster sister; and his current heir, a nervous young cousin named Ernest. The bride is also less than pleased when she discovers that her honeymoon will be more crowded than expected--with Gil's pretty secretary, among others, coming along.

That very night, Julia pounds on her sister's door, brandishing a bandaged hand and reporting a hot-tempered outburst on her new husband's part. Julia is feeling doubt and regret about her hasty decision, but returns to the boat. Then the next morning, before the yacht can depart the harbor, Gil's body is found in the water below--and Phoebe and Eva must discover who pushed him over . . . before the Renshaws' social standing is irreparably stained by Julia's arrest for his murder . . .