Depends what you're hoping to accomplish. You'll burn up most of your glycogen stores swimming before you have a chance to use it as anaerobic fuel in the gym. This is ok if improving your endurance in the pool is more important to you than getting big/strong in the gym. Alternatively, if you were to power-hard in the gym first and then swim after, you'd focus more on your gym gains. Most people I know who are in it for the general well-being alternate every few months.