Have done almost to Mangalore like Fred, the drive a wonderful and you will find a lot of fishing spots old and new ones. Listening to how the dialects switch between languages as you drive further down south is very interesting, you can understand more the similarities between languages which you would not otherwise. There are motels/hotels, eating place and markets all along the way so you do not have to worry as you drive pass if you want a break or anything.
I was on the phone when I read this post with a friend of mine from Kerela, so I asked him, he said he has done the drive several times though he is not a fisherman, he is one who likes the outdoors and said that the drive all the way to Kerela is superb, wonderful scenery etc.
Enjoy your trip mate.