Water temperature will have very little impact on your NOx emissions. However, some things that cause high NOx will also heat your water unusually, which is why high water temp and high NOx are often seen at the same time.

Combustion temperatures are in the 2000-3000 degree range at the instant that NOx forms. If it gets 500 degrees hotter than it should, you will see high NOx. Whether the water on the other side of the metal wall is 190 or 210 degrees will have very little impact on NOx formation.

Interestingly, a stuck airbox thermostat, which only heats your inlet air a few extra degrees (relatively speaking) CAN impact NOx. Go figure.

I don't think that your car has EGR, so the only other things that can impact NOx dramatically are timing (more advance = more NOx), fouled reduction bed in your 3-way catalytic converter (need a new one), or a mixture that is lean under load, caused by any one of several things, most likely the O2 sensor.