Has anyone ever done this themselves? Im talking about polishing them externally for appearance purposes. Im sure its time consuming and tideous. But I think it would look nice to match a polished intake, blower, and stainless headers.

Sounds like torture to me! I've done a few small parts, and everytime I say I will never do it again. Working all the surfaces by hand will be brutal.

It would look pretty awesome though

I did when I installed my new Darts. (just the front face of them) I just used Simichrome and my 7" bench buffing wheel. I do not plan on maintaining them though. You can see the shine on the head on the left.

I polished a complete dodge 727 transmission case once. Took about 10 hrs. Would never do it again.

Eastwood sells a abrasive roll kit for a die grinder. It has rolls in 80, 120, 220 and 320 grit. All of the cast area on the head need to be ground down with the last step being the 320. Above 220 grit you will need to use some type of lubricating wax to keep the abrasive from loading up.
Once the sanding is done, you will need an assortment of polishing buffs and rouge.All of the above is available from Eastwood.
I did a set of 302 Ford heads for a customer's street rod. They looked great when they were finished, but it is a ton of work!

They will run slightly warmer too...

thats due to having less surface area because the peaks and valleys increase surface area it can also increase air turbulence which as anyone who works with intercoolers knows helps cool air.

But its doesnt take and engineer to know that!

How does polishing heads on the outside create air turbulence?

I don't know about the "turbulance" factor, but I do know from experience polished air cooled V-Twin Motors do run hotter than non-polished aluminum motors. This is due to the reduced surface area of the castings. And on the other side the V-Twin motors that have been "diamond cut" run cooler becasue each cut adds surface area. This effect is amplified on motorcycle motors becasue they are air cooled, so would smoothing and polishing aluminum heads on a watercooled motor make that much of a difference? I doubt it would make any more difference than if they were painted..... Personally I wouldn't do it, sanding and polishing aluminum is a pain in the butt, not to mention a pain to keep looking nice. Assuming they are cast aluminum heads, cast aluminum is very pourous, so they will show water spots very easy and will require a lot of work to remove spots and stains. I don't know where you boat, but if saltwater, the salty air itself will dull the polished surface quicker, making more work for you to make shiny again. Just sounds like a lot of work for something that doesn't make the boat faster or louder.......

If you do decide to sand and polish your aluminum heads (or any aluminum) make sure to wear (at minimum) a dust mask. I would suggest a filtered respirator. Especially if you will be using high speed tooling. Aluminum dust has proven to casue very nasty side effects which include, but not limited to, Alzheimer's, and numerous respitory conditions.

Those aren't plastic nipples are they?