Until someone more knowledgeable, not hard, comes along I'll kick off with this.

I reckon the guy in the foreground, armed with an Auxilia styled shield, a spear and sword would do for a marine. Earlier period marines seem to have relied on shield and helmet for protection, presumably because it was easier to move around the ship in, and less likely to kill you if you went into the water, than adding chain or scale.
The guys behind would probably do for sailors or oarsmen.
Some ships are said to have been a blue-green colour as primitive camouflage so tunics might have been a similar colour.
To the best of my knowledge, information is sketchy. So my advice is, go with what looks right to you and dare anyone else to prove you wrong. Remember, absence of evidence isn't evidence of absence.