A flat port will work for simple underwater video, but the air-to-water interface changes how light bends, so you should expect refraction-related distortion and a change in apparent field of view. That’s why underwater housings typically use either flat ports or dome ports: flat ports are simpler, while dome ports are used to better control optical effects, especially with wider lenses.

You do not necessarily need an extra corrective lens just to make a basic housing usable, particularly for your goal of fish identification. But if you move beyond a simple fixed-focus camera, the port design matters more. Different cameras and especially different lenses can need different port arrangements, because the optics seen through the housing affect image quality and framing.

The bigger challenge is usually not optics but pressure and sealing. Underwater pressure rises quickly with depth, and that force acts across every surface of the housing. Making a box that stays watertight and remains usable underwater is difficult, which is why commercial housings are expensive.

So: for shallow, experimental use, a flat acrylic port may be enough; for better image quality or wider lenses, expect to look into dome or lens-specific port designs.