Combine the rice and water in a saucepan and bring to a boil. Reduce heat to low, cover and steam for 20 minutes, or until rice is tender and the water has been absorbed.
In a medium bowl, toss together the lettuce, carrot and tuna. Pour the olive oil and vinegar over this mixture and season with salt and pepper. Toss to coat.
When the rice is done, cool for about 5 minutes then toss with the salad. The warm rice will wilt the cos lettuce slightly. Taste and adjust the amounts of vinegar and oil or salt and pepper if desired. Serve immediately.