That sounds more like the obsessive part of OCD, at least based on your description. The important piece is whether he believes his thoughts are the truth, or recognizes them as just thoughts. Obsessions are also common in ASDs, but they're usually referred to as perseveration, so it's not necessary to start worrying about handing out extra diagnoses. Actually, just obsessing over one thing, especially as a child, doesn't necessarily warrant any diagnosis...kids can have weird thoughts sometimes, and sometimes go through rough patches that don't necessarily lead to any mental disorders. If it's part of a disruptive pattern, then you should consult a professional, otherwise try to relax.
The problem with obsessive thoughts is by trying to control them, we often get stuck thinking them instead. The classic example is, "how many times have you thought of a yellow truck in the last ten minutes? Ok, now try really hard not to think of a yellow truck." So the trick instead is to stop fighting them. As a parent, you can help by just not making a big deal of it. If he's scared and worried about it, just talk him through it.."Did you ever see anything fill up with water? Ok, so what would have to happen for the house to fill up with water? Someone would have to leave the faucet on, but don't you think they'd notice after a while, when the floor was all wet? Also, when it rains where does the water go? Oh, down the drain, right, so since there's a drain, the house really couldn't fill up all the way with water, huh?" etc. Just walk him through the logic, letting him do most of the work and have him solve his own way out of his worries. Be careful not to accidentally give him a list of things to check (because it could lead to some compulsive behaviors), make it very clear that the chance of this thing actually happening, even without taking any precautions is next to nothing.
You could also make it into a joke, if he appreciates the joke, like "Oh wow, well then when the house fills with water, I'm going to grow gills like a fish!" "Does that mean we can just sleep wherever we want to sleep?" Make silly images with it that he will enjoy and make him laugh, again so he stops trying so hard to suppress the thought, and instead sees it as harmless.
The most important thing you can do, even if you do none of that other stuff, is just relax. Being anxious will only make him more upset, and will encourage him to try to anxiously suppress the thought, which will make it stick more. So relax, model that relaxation for him. There is nothing to worry about, because the house is not actually going to fill up with water. And you don't have to worry either, as a mom, because the thought by itself doesn't necessarily mean anything bad about your kid..
Edit: it just occurred to me, if you do live in an area that is prone to hurricanes, or if he's seen things about hurricanes on TV, it may be worth having a realistic conversation about whether he is in any danger (depending on where you live), and then you can talk about what the family would do if it came to that.
Last edited by EmileMulder on 03 Mar 2014, 11:23 pm, edited 1 time in total.