Just paint the bit the light sees - i.e the sides and stern from boot topping or waterline plus a few inches down to the chine or angle to bottom, paint down the bow to the same level.
That is all you need to do and that is all really. Out of light nothing grows except a little slime so do save your money, buy a litre if you can - more than enough for one coat.
Make friends and you could all share one 2.5 litre tin and save more!
Text of a separate post - sorry Trevor, hit the wrong button in answering this post.