Regency romance duke/marquis falls for his ward

I read this book about 6 yrs ago. Don't knw the title or the author or the year. I am going crazy trying to find this book again, Needle in a Haystack! Here are the details that I remember.
:-Duke or Marquis is an illigitimate son of the late duke. Raised by his cruel uncle and aunt overseas to avoids scandal. Grandfather(maternal or paternal?) still alive but estranged.
:- Girl travels to London and reaches the young duke's home in a pig cart and shown the servants entrance.
:- She is duke's ward and is very innocent. Very Rich Duke falls reluctantly in love but she is in love with another guy and is waiting for him to show up.
:- grandfather pretends to fall sick and dying to force the girl and the duke to marry.
:- There is a scene where the girl falls in a lake while wearing black gown( I think) and does not realise its transparency, duke tells her how this effects him and she is too innocent to know.
:-Ofcourse they end up happily ever after.
Anyone can tell me the name of this book has my eternal gratitude.

Once and Always by Judith McNaught :)

No that's not it. I have read "once and always" and I don't think the book I am looking for is by Judith M. Because I would have remembered her name. But will re read it and confirm. Thank You for taking time and writing:-)

Note: in the pond scene, where the falls in the lake/ or pond, duke witnesses it with his friend or butler. Does once and always had this particular scene?

Is it possible the pond scene might be from another book? McNaught's novel does have the heroine delivered to the manor house in a cart with pigs, and that seems awfully distinctive:

"Jack 'n' me, we're awful sorry, ma'am," he said. "We'll take you on ter Wakefield Park -- that is, if you don't mind us puttin' yer trunk in back with them piglets?"

(...)

Ahead of her, the door of the mansion was flung open and a stiff-faced man dressed in green and gold livery cast an outraged glance over the farmers, the piglets, and the dusty, disheveled female approaching him. "Deliveries," he told Victoria in a loud, ominous voice, "are made in the rear."

Google Books Preview has the full scene:

http://books.google.com/books?id=WxoCyyBbpO4C&lpg=PP1&vq=pond&pg=PA3...

ETA: I'm not seeing the black dress that you remember, but there is a scene with the heroine swimming until she's caught by the hero and his friend:

You haven't done anything wrong? And you weren't in any danger?" he repeated ominously, his eyes dropping to her bosom and it heaved with her fearful breaths. Victorial suddenly realized she was dripping wet and scantily clad, soaking his shirt where her breasts touched him. "Suppose someone other than me were standing on this bank watching you -- what do you think would happen?"

http://books.google.com/books?id=WxoCyyBbpO4C&lpg=PP1&vq=pond&pg=PA1...

I think it Once and Always too. In the book she going into the lake to save the little boy's boat. He is the son of the marquis friend, a vicount, and they find her in the lake after the boy screamed when he lost his boat. She did remove her black dress before going in.

Also the marquis got rid of all her black mourning dresses behind her back making her wear colors which she hates.

Oh wow...I guess this actually is the book I am looking for!!! Yay!! I will reread it and confirm with all of you. Thank you and you all have my eternal gratitude. If I had a pet dog I would give my first born puppies to all of you:-)