LEGO® Batman™ 2: DC Super Heroes Mini Guide & Map Pack Walkthrough Level 2 Harboring a Criminal
Objective 2 - Catch the Joker
A - You can only damage the Joker's boat while it's hearts are red. The boat is invulnerable while the hearts are silver.
B - While chasing the Joker's boat around the harbor, blast two yellow carts that roll along a roller coaster track.