I've recently started drinking plenty of water (8 - 12 glasses of water daily), 1- 2 oatmeals daily, a sandwich or pasta for dinner. Is that healthy?

Eat a balanced diet. The amount of water you drink is dictated by thirst. You do not need to drink more than 4-5 glasseswater/day...Anything over will only make you run to urinals.As for food get a good balance between starches(carbohydertes), proteins and fat.. The goal is to have a healthy weight and get adequate amount of proteins, besides starches and pasta. The sources of proteins are eggs/ nuts/, soy/tofu/fish.
Yes. Sound healthy. But be aware that you want to adopt a diet that you like because you want to follow it. Variety in the diet is also important.