Harima doesn't interact with Youko Sasakara often, but the few occasions where he does, they seem to be on reasonably friendly terms with each other. It isn't just him being mindful of the fact that she's his teacher, and also his cousin's apparent lover. She greets him cheerfully in the New Year's episode, and he his response seems to be friendly rather than formal. Then you remember that Harima had to have picked up his art skills from somewhere, and it's possible that Youko had been helping Harima with art long before she was his high school teacher.
Needless to say, Mikoto and Imadori swimming back to shore from the island was risky. In the English dub Imadori suggests he didn't want to try that at first because he was "allergic to shark bites". They end up making the return swim at dusk...a time that the danger of being attacked by sharks in the ocean is more likely. Lucky for them, there didn't seem to be any sharks around, but if there had been ... >shiver<