My tomato sauce is very watery
Yes, cooking it more to evaporate off some of the liquid will definitely help. This is called reducing a sauce. A moderate simmer would be the appropriate temperature. You want to see occasional bubbles but definitely not a rolling boil. Stir it occasionally, making sure to get the bottom of the pot to avoid any scorching. It is possible to have it be quite liquid on top and rather dense in pockets on the bottom, which can then get well above 212 F and reaching the point of burning.