I wouldn't use latex enamel. Oil based would be much more durable IMO and will stand up better to the salt water. Latex will peel and generally doesn't adhere well to glass.
Latex works fine. I would NOT use oil because the solvents could be toxic. Latex is non-toxic, and releases water when it dries. Doesn't matter what time of day you do it. Do a couple coats a couple hours apart.
A medium dark blue definitely brightens the tank more than a black background. But black makes the colors in the tank stand out a little more. I like blue.