The Stepmother by Diana Diamond, paperback

Near perfect. 

Synopsis:

Sixty-five-year-old multi-millionaire Steven Armstrong tells his three children that he is going to re-marry. But his new bride isn't the stepmother they have in mind. Charlene "Charlie" Hendricks is his vivacious, thirty-five-year-old personal trainer, the single-mother of a teen-age daughter. To Steven's midlife children, she is nothing but a gold digger...

Days before her wedding, Charlie, training for a triathlon, decides to go for a swim in the Atlantic Ocean. Out of nowhere, a mysterious figure on a speeding Jet Ski shows up, making an attempt on her life. The children head the list of suspects--but which one plotted her death?
Then, after the wedding, a second attack backfires and mistakenly kills her husband, making Charlie a wealthy heiress. Now it is the children who accuse her of murdering their father. Is Charlie a ruthless fortune hunter, or the innocent victim of her husband's greedy heirs?