Alpine 9886M head unit. The alpine Transom mount remotes aren't the best as they're wireless and thus have a bit of dealy between input and execution of commands, but the Ipod interface and the sound quality are top notch if the the top notch. Plus the RF wireless floating remote is really handy if you do allot of coving.
Polk MM Speakers, specifically the Ultra Marine or UM variants. All polk speakers are Marine rated, but the UM takes it one step further. They are very durable, sound amazing, are both UV and saltwater rated through ISO exposure tests. I've been a professional Installer for ten years, and between the three installers at my shop we have 45 years of experience. Kicker, Alpine, Jl, Clarion all make great speakers, and I have installed them all; but the Polks are just a cut above. Bazooka Probably makes my second favorite speakers, but not as attractive.
Your also gonna need at least one amp, depending on your battery situation, as well as your listening preferences(what type of music, how long etc.), and available mounting location, will determine what style of amp. Class D is more efficent, but you sacrifice a little sound quality and you pay more for a good class D. AB is gonna sound better, but use more power and take up more space.
The amps I would consider would be JL, Alpine PDXM, Polk, Kicker ZXM or IX. Again depending on where and how your boat is stored, where the equipment is mounted and how well you take care of it, Marine Head unit and amps may not be necessary; however marine speakers allways are a good idea