Given a battle where you used all of your Boiling Water PP (+PP Ups), you could expect to burn 4 of your opponent's Pokemon (7) with the attack. by the most conservative possible statistical estimate you will Burn an opponent on your 4th(120%), 7th(210%), 10th(300%), 14th(420%), 17th(510%), 20th(600%), and 24th(720%) usage of the attack.
So the real question is, can you expect your Bulky Water Pokemon to attack three times per battle (90% cum. chance to inflict a burn.). If you can, Boiling Water is an excellent choice. Otherwise use Surf or Hydro Pump.