You don't choose a target for Boilerbilges Ripper's ability at the time it triggers. Rather, a second "reflexive" ability triggers when you sacrifice another creature or enchantment this way. You choose a target for that ability as it goes on the stack. Each player may respond to this triggered ability as normal.
When this creature enters, you may sacrifice another creature or enchantment. When you do, this creature deals 2 damage to any target.
