Are you *sure* you don't want a different boat? Because the costs involved to beef up a production boat of any make from anticipated coastal use to bluewater could be ruinously expensive...and you still wouldn't have a bombproof boat, relatively speaking.
Allow me an example: I had a friend in my teenage years who spent a summer putting a 427 cu. in. Chevy V8 engine into a Vega. Oh, how he worked. Finally, he was done. We all hopped in and the rumble of the engine was stunningly loud. So was the sound of the transmission exploding after about 15 minutes at a mere 75 mph. So was the sound of a body panel flying off from the ridiculous vibration. Lucky I had cab fare. It turned out that while you can convert a Vega into a hot rod, it's still a Vega, and it won't last long as a hot rod.
See the full keel/fin keel thread for some greater depth.