If your heart rate isn't up, you aren't working it hard enough. I used to teach water aerobics and there are people who would come and sort of jog in the water and not do much else and then there were people who were pounding the bottom of the pool deck like crazy and pumping their arms. If your teacher happens to be the lazy type, just follow what move she is doing and add intensity to it. Don't worry about looking ridiculous, everyone does! If you really gave it your all abnd still didn't raise your heart level try taking your arms out of the water and reaching them up while jogging or jumping in the water. That seems to get mine up there quickly.