It’s a good cardio exercice, but if you want to gain mass, you’ll need to lift weight. No one ever got big from rowing, you might get some shoulder definition, that’s about it. If you do more than 30-45 mn of cardio, your body stop burning fat and start burning muscle. Go to the gym and lift 3 to 4 times a week. You need to do heavy, short sets and consume 1gr. of protein per lbs. a day to start. if you weight 150 lbs, you need 150 gr. of protein. good luck

rowing machines are great. but dont limit yrself to just that. read tips on gaining muscle with natural proteins and more on some great exercises on this site