If only he were wearing his collar, she'd know at first glance.

Andrew's eyes go wide as he recognizes her too late to pretend he's not here and let her wander along her way. Situated where he is, too, a getaway into the water is a little more involved than just ducking under the surface. This is borne out when he gives an experimental little thrash of the tail as he's thinking about the logistics of making his escape, causing the surface of the inlet to ripple.

"Lucy!" he squeaks, trying (and failing) to look pleasantly surprised. "Fancy seeing you here!"

Rassilon's knickers, will she be able to tell? No, no, that part of him is under water right now, but he'd kick himself for calling her over if he had any hind legs.