Corrected entry: On page 221, it says "If anybody noticed us - five kids dripping wet and looking like they'd just had a fight with a monster - they didn't let on." But Percy and Tyson are BOTH sons of Poseidon, and sons of Poseidon don't get wet when they're in water.
Correction: Not if they don't want to. I suppose it would've been a lot easier for them both to not be dripping wet, but as to not set the five apart, Rick probably just kept them all wet.