You are right, you should get some waterproof marine speakers for it..i would recommend the sony or clarion brands.
Find a pair of Realistic Minimus 7s used in good shape if you can. Should cost only $50 or so and they are top notch.
Metal enclosures might be better but wood will work as well as long as there is no direct exposure to water. I've used a pair of &s with wood enclosure outdoors on my deck under cover for years and they are still going strong.