I lap swim everyday it seems to give me a great workout. what i did to make it a little more difficult i bought some hand paddles and also a weighted belt. This give me max resistance. I also bought a waterproof ipod shuffle so i can listen to music while i swim bc its soooo boring after a while and hard to stay motivated.
Just switch it up when you swim, go online and get a couple set schedules and do them you will get a great workout.