Having built several boats, I find that the boats float too low in the water to put an attractive line on them by floating them. Most waterlines are only seen when the boat is out of the water on its stand. Therefore, I put a waterline on as if it were a full size boat. It will be below the water when floating, but who can see that anyway unless you are at water level with a diving mask!
With the waterline at eye level, sight your work with a straight edge. Normally the waterline is slightly wider in the bow and stern if it is a stripe. If this is not done it will appear thick in the middle and thin in the ends.
To get an idea of a consistent width, find a model striping tape to use as a guide and tape to the upper and lower edges of this tape. Then remove the guide tape and paint the stripe.
I use either fine line 3M tape or Scotch Tape to do a painted stripe. The stripe should be done before the topsides and bottom are painted or you will be able to feel it as it is on top of the other paint. This is slow!