I used the paid version of "Add Watermark" by AndroidVila on Android. There is a free version too. IOS has a similar app. On myiPad, I use Scribeit (also a paid app).
Whichever app you choose, always make sure it can store or remember a frequently used text. This is important and time saving out on the field and when you have to work under the glaring sun. Also important to have freestyle one finger positioning by dragging.
Also you may like to have the apps insert a graphic or logo when necessary. Both the apps I mentioned can do that but I rarely use the feature.