Scratch build every time. I like to build something that is similiar to a specific boat/ship but not have to be meticulous so the 'critic' can't say, '???? does not have that shape hull/sails that big/is not coloured like that, is bigger than that, etc, etc, etc. A perfect example of this was my last build, a 5 foot plus aircraft carrier that was named HMS Invincible because it looked like it. The 'pain in the a** critic kept saying, 'Invincible is not like this, that, does not have, does have, etc.' After several weeks of this I said to him that it's name was not Invincible it was HMS Indifference, so now I can do what I like with it. Shut him up for sure.
My last 3 builds were:
An adapted pink Barbie plastic boat that had 2 motors mounted on pylons and small propellers, including Barbie in the boat and Ken on skis - a real head turner
An airboat made from a sheet of foam and 600 motor - really fast
HMS Indifference, aircraft carrier with all bells and whistles, music, radar, etc. This is featured here on 'Your Models' and looks brilliant on the canal. I often follow within half a metre to the rear of barges and the kids love it (not to metion the adults)