How can I make treasure ships appear?

To make a Hammer Bros. turn into a coin ship:

1. Ensure there's a Hammer Bros. left on the world map
2. Start a level and finish with:
   1. A multiple of 11 coins (11, 22, 33, etc.)
   2. A score ending in 10 (2010, 3410, etc.)

Each second left on the clock gives you 50 points. That means you should end with an even number of seconds left on the clock if you have a score ending in 10. If you have a score ending in 60 you should finish with an odd number of seconds on the clock. Anything else and you're out of luck!

There are also white mushroom houses that appear if you get every coin on a particular level in each of the first 7 worlds:

1–4: 44 coins
2-2: 30 coins
3–8: 41 coins
4–2: 22 coins
5–5: 28 coins
6–7: 78 coins
7–2: 41 coins

I am assuming you mean turning a wandering Hammer Bro into a white treasure ship. From Gamespot:

Morph a wandering hammer brother into a treasure ship. To turn a wandering hammer brother (the ones that walk about on the world map) into a white ship full of coins, collect a number of coins that is a multiple of 11. Make the tens digit in your score match the multiple of 11. Stop the timer at the end of the stage that you are playing in on an even number.One combination would be: 11 coins, 9,310 as your score, and the timer at 104. This only works in worlds 1, 3, 5, and 6.