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Yeah not sure about fishing there. I've been diving there (out of Nha Trang) and there wasn't much to see. Everyone else on the dive was impressed but i guess I was just spoilt doing most of my diving on the GBR.
Only fishing i got to do was help pull in a big sein net the locals had set out on the beach at Mui Ne (between long island iced teas). they do take and eat just about everything from the ocean there, doesn't seem to matter how small. It has undoubtedly changed the size distribution of fish populations in the region.
You might be lucky if you can find a charter which goes offshore.. look for expats from other countries working there though, not sure if i'd trust a vietnamese charter.
You'll have a great time though, it's a lot of fun and a very easy country to get around in and cheap.
As mentioned stick to the fishing park or go to the fish markets, there is some game boats around Da Nang but I didn't hear of any success. You will have a great time, my wife and I did the same thing for 3 weeks and loved the North and Hoi An.