How to use the boats as Batman in Free Roam Mode
Assuming you're talking about the lake part in the Iceberg Lounge, to cross it you have two options:
- As mentioned in the comments, use the line launcher to travel across it.
- Toss a freeze grenade in the water and hop on it to create your own raft, you can then pull yourself with the batclaw.
At least three, actually:
Grapple-boost wall-stall (jump, cling), as well as line-launcher and freeze-blast ice-floe raft.