I can't speak to saltwater, but I can speak to drying time. I use Krylon Fusion a lot (75 gallon tank, and two outdoors ponds). If you are trying to hide or blend, use the flat. Satin is ok in a pinch. But the glossy will pick up highlights from your tank light, and I think it is distracting. If you want ultra-flat black, look for the camoflage series of Krylon Fusion. Good stuff.
The longer it drys, the better it stands up in areas of high water movement. The more time you can give it the better, regardless, but if you know the painted item will be near or in a current (filter outflow, for example), give it as much time as possible. I like a week.
And be aware that there is a small minority of plastics that it will stick to, but never dry on. For me, this has happened with power cords for powerheads. The powerheads dried as expected, but the cords NEVER did. No idea why. But this is really rare. I have used it alot recently on PVC
, and have been really pleased.