First time canner here …. Do you keep the water at a boil or simmer

And here's a tip you'll find helpful, no matter what you're canning - but especially for this recipe, which require a full 45 minutes: Keep on your stove a kettle of water that you've brought to a boil and then maintain at a simmer during the 45-minute hot water bath. A lot of water will evaporate so you'll need to top it off regularly to keep the water level at a good inch above the tops of the jars. When the water in the canning pot looks like it needs to be refreshed, crank the heat up under the kettle to bring it back to the boil, and then carefully top off the water level using the boiling water from the kettle. ;o)

Boil! Make sure to leave adequate headroom in the jar, and make sure they are completely submerged. Boil for 45 minutes (as the recipe says) - the boil time for canning tomatoes is much longer than for jams or jellies which only take 10 minutes. Good luck!