calories burned in exercise question

I couldn't find any resources but here's the story,,,,

I take water aerobic class. this is HIGH impact class -- we jog in water the whole session. Water aerobic gives me a bit more than 300 calories per hour which seems very extreamely low for this type of class.

So,, in case of high impact, do I put in low water aerobic calories number or higher water jogging number?

Please don't ask me about getting HRM,, I can't afford it and there's really no proof it works.