Well, that pretty much nails it. The mead is sweet because the yeast reached it's alcohol tolerance limit of 14%. In theory it should be possible to add a different strain of yeast with a higher alcohol tolerance, like EC-1118 which has a tolerance of 18% and get it to convert the remaining honey, but in practice it might be difficult to get the yeast to start up with that much alcohol already present.
If you add water, it will dilute the residual sugar to make it less sweet, but it will also dilute the alcohol, the flavors and the body. You'll end up with something watery. And it's been too long for the existing yeast to take off again, so if you want further fermentation you'll need to add fresh yeast.
Best bet might be to make a new batch from scratch and blend it with your existing one after both are done. If you do that, don't put in any more honey than the yeast can completely ferment out, so it will end up totally dry. With that upper limit in mind, you can also calculate how alcoholic you want it to be so that when you blend, you can hit any target strength you want (somewhere between the strengths of the two batches).