While on a cruise, there should be PLENTY of Level 1 and 2 options. Salads, fruit, veggies, lean proteins are generally always available at the buffets and dining rooms. Ask for salad dressings on the side and make special requests when necessary!
Tips....fill up on fruits/salads first. Grab apples to take back to the room to have on hand throughout the day. Pack unsalted raw almonds for excursions as well as a water bottle. Use fruit squeezed/olive oil as dressing on salads, ask for rice/potatoes/breads to be left off plate and add extra veggies in their stead.
And lastly...avoid the elevators and use the stairs...that can be a great workout.