Unless you are already a good swimmer, I don't think that swimming is going to do a lot for you. I used to swim a lot and liked it okay. It can get pretty boring looking at bottom of a pool, if that's where you are swimming. Still, I was dedicated and did it regularly for a year or two. However, unless you get your turns and can your breathing down to where they are efficient enough to really kick it, then, IMHO, I don't think you can get the same aerobic workout as you could running, biking or whatever.
Still, if that's what you enjoy, sounds like that's what you ought to do... maybe just BECAUSE you enjoy it! As I recall, I thought I read that water insulates the body so you don't burn as many calories for the same amount of effort. If that's true and you are trying to gain, it may be the ticket for more than one reason.
On the other hand... I did have a "swimming instructor" friend tell me that the worse a swimmer you are, the harder you work so it's actually more work to move the same distance. I understand the logic but didn't experience it myself since I've been a decent swimmer all my life so I don't know about that one. I see a lot of really well built swimmers... the competitive ones... but I don't see people dropping the lbs like crazy from flailing up and down the lane. She also taught water aerobics.
Q