Marine Grade paint works best most likely, but it is probably overkill. You could also use Epoxy paint since it practically indistructable once it sets but that is likely way overkill. Personally, I would use a nice coat of primer and then an exterior paint. It will hold up just fine. People who live near the ocean basically use the same thing when they have to paint and it will usually last 20+ years. You might need to touch it up in 10 or so years, but I doubt it. Sun and temperature variation is usually what does in paint. You will likely have neither. The paint and primer will be a great moisture barrier for the wood so I doubt you will ever have to do anything.
Nice work BigD! I would recommend exterior grade latex paint over a couple of layers of exterior latex primer. It is easy to work with, easy to clean, and being exterior grade, it is very water resistant. Before you paint it, I would also recommend getting the paintable latex/silicone caulk and seal all of your edges, corners, and nail holes prior ro painting. That will help keep the wood from warping and the nails/screws from rusting and failing. BTW, using white semigloss for the interior of the hood is best since it will halp reflect as much light into the tank as possible.