This is not something you will need to concern yourself with if you solely use your GPS TomTom watch for its basic functionalities. But, when you log into TomTom watch services on your smartphone, some of your data will be used up. However, thankfully, it will not use very much. Say, for example, you use any service for one hour per day, it would approximately take up 7 megabytes of data per month. To put this in perspective, that is the equivalent of streaming two music tracks per month.
However, when you travel abroad and utilize their services, you may encounter additional data-roaming charges. The only way to be certain is to check with your mobile provider and see the rates they charge for using the internet abroad. Even if it increases it a little, it will still not take up enough data to concern you.