You are getting some good swimming time! I did a mini-triathlon once wherein one leg cramped up and I had to let it hang and only kick gently with the other leg. Luckily the swim was only 1/4 mile!
Yeah, I now often have this feeling that I could just keep on going and going with swim. However, as with running, I'm quite slow, especially on longer intervals.
Lately I have tried actually timing myself and working harder on the shorter intervals, which may help.
On the other hand, I'm just grateful I finally got continuous breathing figured out.
As for cramping, it seems as though I only get cramps at certain times: 1. after I've been swimming for at least 45 min or so, 2. kicking off the wall, or 3. kicking hard while wearing fins (my fins are even shorter than most "short" fins out there, but they still extend my feet nicely and work my legs harder, however they pinch my toes after only a few laps, so I have to be careful with them)
I have been able to "shake" off a cramp while swimming a few times. In general, though, I've had to stop and massage/stretch thoroughly before I can go on.
I'm amazed you were able to continue swimming with a hamstring cramp. Hard core toughing it out.