Please people, there's no need to start with any namecalling because a newby asks something that is very obvious to the rest of us.
The problem with submerging any markers is that water will enter. Be it throught the barrel, or any other means, it doesn't matter.
Depending on what you plan to do with it, it may or may not be harmfull.
If you plan to submerge the marker during play, I would strongly recommend against it. Once water entered the barrel, and came in contact with the paintballs, the paintballs will start to swell up, pretty much rendering your marker useless, because all that you will be shooting until your marker has dried out, is snot.
If you are interested in submerging your marker or 'hosing' it in order to clean it, there's some markers that you can do it to. Make sure it's not an electro though (shortcircuits and all that). If you get one of those simple blowback affairs (Tippmann 98 for example) you'll be okay(ish). I'd still not recommend it, because of the risk of quicker oxidation (rusting), but it won't do any big damage that'll prevent you from using it again soon.