Well, he did say "try" -- he's aware that Eren is basically a dirt magnet.

But hey, they're having a nice moment, and the water and sun and sand are all distracting and lovely and Armin is...actually enjoying himself, wow. So are the baby Pokemon, making little sandcastles, watched over by reluctant babysitter Meredith.

That gets a bit of a surprised look, then a very Armin-esque attempt at a joke. "That would be quite a commute to the Ranger headquarters every day."