I'm imagining a scene where they were going to get together but something interfered, and Ami retreated from her love for Mako. She put distance between them, causing Mako to feel her absence more keenly, until she could take it no more and confronts her at the pool. Ami dives in, trying to avoid her, but Mako will have none of it and goes after her despite still being in her uniform, not letting the woman she loves get away.