So I set up an encounter in a sewer system with an Epochian Mermaid and Shockbus. It was your standard video game boss affair. The players have platforms they can stand on and fire on the creatures as they pop up, the creatures have access to pipes under the platform allowing them to move across the platform quickly and get some lovely back attacks.
4 rounds in and one of the party members, tired of wack a mole, dives into the 30M deep water. I was able to cobble some basic rules together from the base rules (HC Agility checks for swimming and Willpower for breath holding) which held us over.