SePRO Sonar RTU Aquatic Weed Herbicide can be harmful if you treat the entire pond at once. It is recommended to split applications as directed on the product label.
There are not swimming resitrictions with Sonar or Captain applications.
Penn State actually has a great article on this weed that you can find here with advice on preventing it. SePRO Sonar RTU Aquatic Weed Herbicide would be good to use on watermeal itself.
SePRO Sonar RTU Aquatic Weed Herbicide only kills those weeds that are contacted directly and is typically applied over several days as listed on the label for control. Apply when weeds are actively growing. Best results typically achieved by starting applications just as weeds have started to grow.