Bertholdt does stop, cheeks and ears flushed a little as he glances back at Reiner.

"I-is that allowed?" is what he ends up saying. Not a no, not ignoring Reiner and continuing his little swim. In his own eyes there's a tiny spark of interest.

He's openly admitted how he feels about the power of their Titans, a feeling that extends to the magic here. But if anyone would know about that secret pride, it'd be Reiner.