What the Water Kept. A Claire Harmon Mystery, #9

The ship pulls out of Miami with six hundred feet of steel underfoot and land nowhere in sight for fifteen days. It's meant to be nothing but a celebration - old friends gathering aboard a repositioning cruise for a long, unhurried retirement send-off, days of blue water ahead and no ports of call before Naples. Then, on a night when half the group is asleep and half is at the rail with private worries of their own, a man is found dead on the deck below - and the ship becomes a floating question no one can answer until they finally reach land.
Claire Harmon didn't come aboard to investigate anything. But grief has a way of looking exactly like guilt, a marriage-ending confession has a way of protecting the wrong person entirely, and by the time Claire understands what's actually being hidden, and from whom, there's nowhere left on board to be wrong about it. Fifteen days. One ship. A secret that has waited far longer than that to surface.
For fans of Ruth Ware, Lucy Foley, and Agatha Christie.