Hey!
Now you see why I tell everyone I am teaching to do this. Take some LEGOS or other stuff and make a jig base that puts a #2 drafting pencil at the upper waterline.Then slide the rig from bow to stern gently leaving a nice line behind. Then, Lower the pencil to where the Bottom of the line would be and draw that line too. NEVER let the pencil push to hard or too light .Those lines are where you tape for top and Bottom.There is the Fact that Bill mentions and the jig will help you there.When you look at the lines drawn you'll see what he is referring too!
When you measure from top to bottom do so at directly Midship.That way the lines will be accurate! I do this also. I will sometimes, Using A very flexible plastic rule or cheap auto striping tape, Put this on the top and bottom of the lines. Then Very carefully and not with much pressure, Draw the backside of an X-Acto knife along the stripe. That way, when you get ready to Paint you will have a fine line to follow with your tape.
To make a seasoned stripe (been at sea for a while.) Paint your line Coal Black. Then mist the bottom color slightly over the lower edge. This will give the effect of Salt Water Erosion of the painted surface. If it's (The model )1/350 or smaller don't bother, just paint the line Charcoal, otherwise it will be to stark against the other paints. Also, Based on what Capn'Mac said. When you peint the bottom there are two colors you can use.Good old Testors Rust. Or, The Same, Tinted lighter with a bit of white.
What I do is take an eyedropper (Glass.) and draw about one inch into the dropper, Carefully let as much as you can, go into a mixing cup or palette. Then using a clean dropper(Glass) take about a sixteenth of an inch of the color Insignia White(flat), and drop it into the Rust. Then Mix well, Stir till you have a nice pale shade of rust with NO streaks. Thin the amount you need, transfer to the airbrush cup and spray.