did i really burn that many calories?

okay, so when i look up calories burned for swimming it says that under a vigorous effort, which CC defines as 75 yrds/min would be 729 calories an hour. but for me, i wouldn't consider 75 yrds/min to be vigorous or hard effort so does that mean i actually burn less than 729 cals an hour at that pace?

If you find swimming at that speed is effortless that probably says more about your fitness than your energy expenditure.....  These calculations are really only a best guess so don't read too much into them.   For practical purposes it's sometimes better to fix on an average activity level and go with that rather than trying to itemise every activity individually.  For example, if you're pretty active for an hour or so most days set your activity level to 'very active' and work with the calorie recommendation that comes back.