Armin here - I'm myself for this one, though I have an infiltrator persona. [Oh, right - he pauses as he realizes Percy wasn't around for that.] ...That's basically psychic skill where you can infiltrate while still remembering who you are. It doesn't always work, though, but when it does it's very useful.

Keep in mind that while your dad is Poseidon, he might not be this Poseidon. [He doesn't want to see you get on the wrong side of a god here - though the potential of having one as an ally makes the attempt worth it, in Armin's book, as long as he's careful about it.]