OHW in the Age of Sail
First thing you need to do, however, is use a hex grid. Most sailing games do well with a hex as you can model a sail's reaction to the wind rather well. Wind coming straight on stops you ("in irons"), wind abeam (green) allows you to travel faster, and wind close quartered or aft (yellow) makes a little slower.
So movement would have to change a bit from OHW.
The core concept that you would keep, however, is that combat is simply reduced down to a single die roll. Whether you would roll a D6+2, D6, or D6-2 would come down to how many guns your ship has. Like OHW you would simply record total hits on the unit and once it reached a certain number the unit was removed from the board. (Whether that represented a ship running, striking its colors, or sinking was irrelevant from a battle viewpoint - you were out of the battle in all accounts - but might warrant figuring out if you were playing a campaign.)
If you wanted to go through the complexity of it, you could assign a different number of hits to each ship type. I would not go nuts with it, but you would probably find that the ships with more guns also take more hits.
Ranges, by the way, would be short. More like infantry arms than artillery, to account for the inaccuracy of firing on an unstable firing platform.
Another Neil Thomas concept is that you have to roll for force composition. That could also be done with this variant, substituting one unit type for an appropriate naval one. My guess is that you would want 3-4 ships with D6 and 15 hits, 0-2 ships with D6+2 and 17 hits, 0-2 ships with D6-2 and 13 hits, and 0-2 special ships. What those special ships would be would largely depend upon the scenario. They could be merchants to be guarded, a group of gunboats, an oared galley (Pirates of the Barbary Coast), or a sloop rigged fore-and-aft (as opposed to the standard square-rigged sails of the day).
Just some thoughts to toss out. If I expand on it further, I will definitely write a post about it.