The more light, the faster the shutter speed that can be used, as well as the larger the aperture. You want a large aperture (actually is a smaller hole) to increase depth of field. I also experience a fade/wash out of color when using the flash, but the flash allows you to take crisp pics of moving fish. In the future I will be investing in an external flash, so I can put the flash at a different angle than the camera, I think this will reduce the wash out caused by the flash. If you have an off camera flash, I would say go for it. But either way, I would experiment with both, take some pics with the flash and some without. If I were using a film camera, I doubt I would use a film higher than ISO 200, preferrably ISO 100.
If using a film camera, I would also recommend getting the pics processed at a place that allows you to pick the photos you want to get. Eckerds here in the states does that, that way if you take a roll of bad pics, you don't have to pay for the processing, which is often as much or more than the roll of film was itself.