If you want to talk theme, there is always the background forum. People here want to know if they can field the army. According to theme space marines don't swim, they scare water out of their way. They are worth whole armies of men apiece, they are rare in the imperium.
Please, let's be resonable, this is an army list, not a background book. Sure marines are rare, but in reality MEQs arew around 60% of the armies you come accross. We are debating their use as a list attached to another. Marines cost 2.5 guardsmen. It's a GAME.
As such let's make this debate productive.
I know the theme is against it, but if that was the case we might as well say no Kroot for ANY imperial army, but as I stated before it is obviosly designed with the imperium in mind (hell, they even have quotes to that).
As such I suggest you bring the theme more in line with the rules. Sure often rules-lawyering is against the spirit of the game, but I have not been doing that. I merely ask that you base your arguments on the rule books please.
As such I direct you to the Inquisition forum: Are GKs Just another Marine Chapter. Obviously this lies at the centre of the debate. While technically DeathWatch Kill Teams can be fielded, I have agreed that that is against the theme as they are Marines. Let's consider if the Grey Knights count, or if inqiusitors (radical ones) would thematically fit.
Basically lets just see what is reasonable. Personally I'd let anyone use any list against me as long as he's just trying to have fun and be different/unique. If it's for the sake of exploiting it, then he can go play the other cheese-mongers. I see great potential in an awesome thematic Kroot/Inquisition list, so I'd encourage my opponent to use these rules.
Because when it comes down to it it's just a game and i just wanna have fun <cue 80s music>
Oh, and I appologise if I came accross a bit passionate, I wasn't feeling al that charitable;) I promise to play nice if you do :p