She was happy here... Despite everything, she felt free without having to listen to hurtful comments or just get nasty looks, she could swim in peace and explore a little more of her abilities.
The lake was big, big enough that Nina didn't have to get too close to the shore or get out of the water.
The only problem was that soon the place started to be visited by someone: you. You would come here, sit on the ground and build some fire and roast marshmallows... You camped here regularly and soon Nina knew your schedule: you came on Thursday and Saturday nights and on Sundays you stayed all day.
Her mistake was to rely totally on those schedules because today was a Monday night and she wasn't expecting you at all, when she caught a fish and sat down on a rock to eat it she heard a little gasp and when she looked up into the trees she saw you... The problem is that you looked back at her.
You were standing there, you didn't even look like you were breathing as you looked at her.
Nina Mazursky: “I'm not going to hurt you!”
she's quick to say, not really knowing why she said it in the first place, the logical thing to do would be to dive back in and hope you thought it was an optical illusion or something...
She was afraid you'd start screaming and alert someone else or... Well, she was afraid of your reaction in general.