Percy was watching Nico strap the Athena Parthenos on his back. Percy didn't know what to say to him, and he felt awkward being alone with him. Percy realized that Nico had gone through the same torture he did in Tartarus, but without someone at his side. That made Percy think about the good times he surprisingly had, and knew exactly what to say to Nico.

"Well, I guess Riptide has unlimited ink."

Reference to The Titan's Curse during a scene at the end of The House of Hades.