I normally swim 2-2.5km per session about 3 times/week. I reckon on an average of 30 secs (or just under) per 25m, or 40 mins for 2k. Not usually breathless after that so I would consider that light/moderate. I would consider fast as an average of say 22 secs/length or sub 30mins for 2k combined with a level of physical exhaustion--just my ten penneth worth

I've been struggling with the same problem. Being an engineer, it bothers me quite a bit that I can't "measure" how many calories I'm burning - my type-A personality can't handle this unknown when it affects my goals. I'm a bit slower than you - but I bet I'm less efficient (hence burning lots more calories to do the same work) .

Here's what I've found:
I swim 2k (doing intervals) in just under an hour - and I'm pretty tired when doing it. My HRM says I burn upwards of 1600 calories, but I don't really think that's real. I tried a Poolmate and it lined up pretty closely with "light/moderate" for calorie burn - so I've been going with that. I've also checked websites on calories burned while swimming and they also confirmed "light/moderate". The wrinkle in all this is that I'm losing more than my goal weight per week. I'm very rigorous about entering ALL of the food I consume (type-A personality again), and even though I almost always have 0 calories left at the end of the day I end up losing a bit more than my goal each week. Yes, I eat my exercise calories back every time.

In other words, I seem to be underestimating one of two possible things:
1) I have a desk job, but I do get up pretty frequently to cruise the office to see people - so even though I've entered sedentary as my lifestyle in my goals section, maybe that's not accurate.
2) My calories burned during swimming is higher than "light/moderate"

So the the conclusion I've sort of come to is that maybe using my perceived level of effort is the best measure when combined with MFP's exercise entries. If I feel like I'm working really hard and my muscles are a bit sore for a few hours, then likely I was "moderate/fast" but if it felt pretty easy that day then "light/moderate".

At the end of the day (over 3 months of thinking on this topic) I figure I'll continue to enter "light/moderate" but I won't sweat it if I go over my calorie limit for a given day when I swim. Likely I'm continuously underestimating my calories burned - but if I keep to within 100 or so of my goal I'm certain to lose weight. Additionally, at my weight I can afford a few extra calories burned without any danger of going into starvation mode (something you may not be able to do) - my calorie limit per day is 1400 (still well above 1200 / day danger level).