We did hull stripes with this
paint last summer, using the roll and tip method. It has worked out well, took 3 coats (no primer) putting flag blue over white.
I added a reducer that helped it to "flow" out, leaves a smoother finish - (don't recall the name/brand) If you use a similar product be careful you don't overdo it.. it really wants to run if you do.
When you first roll it on it LOOKS LIKE HELL!

The foam roller leaves a spotty, bubbly looking mess. But when you tip it with a foam brush, all you are left with is a bit of streakiness, which flows out quite quickly. The results are really quite good.
Try to keep a wet edge, only roll about 4-5 feet at most then go back and tip it right away.
Getting the
line straight now that you've sanded is going to be tough. The laser trick works if the boat and the laser are perfectly level, or if the boat and the laser are in the same plane in all axes... Leveling the laser is no problem. Knowing if your boat is truely level on the stands is another thing altogether. Also remember that the boot stripe thickness will have to change as you go around the counter in order to have the same "visual" height all around when viewed from a short distance. (eg a 2" boot stripe can be 12" wide under the transom on boats with long flat overhangs - this is less pronounced with more traditional designs.
If you ever do this again, score the hull lightly every couple of feet along the stripe(s), those marks should be visible even after sanding.
Good luck!