You can dilute it with water, but that's usually done just to thin out juice that's too thick. By the time you reduce the nic to something that makes any difference, you'll wipe out the flavor and it'll be all runny and not worth spit.
Cartos are always less intense than atomizers. Just take shorter hits. There ain't no rule saying you have to take a massive hit. Try holding it in your mouth longer and inhale more shallowly (hm.. I guess that Is a word.) Vape it more like you'd smoke a cigar or a pipe. A lot of it is pacing and technique.
If you absolutely must dilute it with something, at least use some VG. You can get it at any decent drugstore, or even Walmart. Ask for vegetable glycerine, USP so they'll know what you're talking about. It'll still do a big hit on the flavor, but at least it won't turn your vapor into steam.
Also, when you're using the condoms to fill, if the first one doesn't do the job, pull the carto out and fill it up again. It usually takes me 2 condoms, each about 1/2-2/3 full to completely fill a carto. One condom is not enough.

Whatever you do, don't underfill on the first fill-up.