Thanks very much everyone for the advice, xamkrah for the drawing, Kamra Obscura for the comparison shot.
Kernuak- I'll have to read up on grad filters and I'll try the shot again using your framing suggestions.
To answer your question, Keith- PhotoShop Elements but all I did was level the shot and re-crop slightly to eliminate the resulting whitespace. I tried to mess with the colours but didn't like the result. What my uneducated, unsophisticated eye finds pleasing about this photo is the way the blues and purples transition to the pink and yellow. So although I hear your comment, DB, about dead space, I'll have to take another look because that's where the darkest blues originate.
Oh, and BTW DB- not a SeaRay, a Chaparral Signature 240
Which, because it needed a new cockpit cover and boat gas is now $1.55 per L at 30 litres per hour- is the reason I won't be getting a 5D3 anytime soon. (Actually, probably not even a new Rebel since that dock needs a new ramp, too, but I digress...)