Raising the temperature of a body of water by even a few degrees can have disastrous consequences; from outright killing species, to producing algal blooms that deplete oxygen levels (and then kill species). I mean, think about it. Water resists temperature change much more than air, so a sudden increase is bad news to creatures that just aren't made to deal with it. Also, a recent study has found that increased carbon dioxide levels are making marine life more susceptible to fluctuating temperature and oxygen levels.
But, of course, just one place in the ocean using this method isn't going to have that much effect. It's if and when this cooling strategy starts to catch on that we have to worry about affecting our environment, and weigh the consequences of air conditioning (fossil fuel emissions) versus heat pollution.