Dean's ultimate revenge (we only saw the first part!)
Even after Joanna gets her memory back and returns to the yacht, this is still part of Dean's plan.
By this time he has successfully charmed and woo-ed Joanna that she is deeply in love with him and the 4 boys. She has discovered a new way of life that is more fun and loving than the one she has went back to. Of course she will change her mind and come back for him.
Dean knows this, but needs to give her enough time to calm down and start missing him (after being understandbly angry at first). He waits long enough, whilst pretending to his family and friends that it's all over with Annie. Once enough time has passed that Annie will be starting to have a change of heart, he goes after her.
Annie chooses life with Dean, and they swim out to meet each other. Annie and Dean get married.
After a suitable period of time, Dean divorces Annie and gets half of everything she owns, and half of her vast wealth.
With his new wealth, he and the kids buy themselves a mansion (employing the ex head mistress as their maid after she got sacked for her mistreatment of sick children). They leave Annie heart broken, with Dean uttering his parting words "bet you wish you'd paid for the closet now, b***h."