Delphinus/son of skyline I think you should be safe with either a spar urethane or spar varnish. On my fresh water tank I have used spar urethane to coat the pine cribbing and it has held up well (under a 1000W Mh bulb). The urethane is essentially a plastic. There is a water based version of this called verathane...but it is not nearly as tough. I expect the marine paints that the store is refering to are called antifouling paints. These come in a number of flavors and are designed to prevent maring growth on the hull and most contain copper (amoung other things).

I have had a sailboat for years and I would use a paint made by Sikkens. These paints are simply the best I have ever found. They are somewhat elastic so they don't crack and have a UV guard. The make a marine varnish called cetol, and also some house paints. You can get these here at windsor plywood.

The trick to these paints is in the application. I would put a coat on, use a wet sand paper and sand it smooth as the paint raises the nap on the wood. I then would apply apply another coat and sand and then a third coat. You are trying to get a very smooth surface. This is effective on even particle board.

We use Tremclad Rust Paint (Medium Blue) here at the Aquarium Store (Aquiarium Illusions) for the outside of the tank. If you ever need to get it off, it just scraped or peels off...

Works well.