Any water heater built in the last 10 years will be significantly more energy efficient than your stove. At the same time you might want to consider a number of other variables if you are really into crunching the numbers on this.
1) you mentioned that your pipes are not insulated but didn't say how much pipe the water travels through before reaching your kitchen sink, or how close that pipping may or may not be much colder areas such as exterior walls, unfinished basements etc. Without knowing that kind of information no one can really say how much energy you are loosing in between your water heater and your sink.
2) The time of year, and whether you are trying to heat or cool your home, matters a great deal if you are thinking about how either choice plays out in terms of CO2 emissions. If you are boiling water in the winter, a lot of the energy that your stove "wastes" while heating up a pot of water goes into the kitchen's air and is helping to heat your home. That might matter a lot if you have a pretty wasteful forced air system attached to an older furnace, or might not really help if you have an efficient radiant floor set up or a geothermal heat pump etc. Conversely, turning your stove top on in the summer will make your AC to run, which in turn compels the nearest coal or natural gas power plant to burn more fuel. Make no mistake, even if you have solar panels, everyone who is tied to the grid is consuming at least some dirty energy at night, cloudy days, and any time appliances are drawing more than panels provide at that precise second.
If you are really concerned about this you also might want to look at a solar water heater on your roof which is way cheaper than PV solar panels, eligible for a 30% tax rebate, and will drop your water heater bill by anywhere from 50%-80%