Yes... Naval Infantry. Not Marines -_-... I've seen a few mods that have 'em or something similiar... but nothing entirely universal as far as I can tell...
The idea would be to make a universally availible unit such as Line Infantry, but called Naval Infantry or something like that anyway... probably availible for recruitment from shipyards... etc... and maybe cities that have the naval academy building or w/e its called. The unit would basically be line infantry or or mix of line/light infantry... the range and numbers of line inf... but with better melee, light infantry classification, and looser formation.
It makes sense that such a unit should be in existence. After all... for quite a while marines werent considered the most important part of a naval landing, but armed sailors were... and sailors were trained to function as infantry until 1900+... then marines took that niche from them...
[tried posting as a pic but just showed a red X...]
Some Info about it... (yes it covers a later period... so what...): http://www.history.navy.mil/library/online/naval_infantry.htm
Anyway... an idea for a new unit... not faction specific (though some factions could have special versions of it... elite units are never a no-no)