Just a quick update, but I found a pond maintenance guy and he came out 3 times to apply a chemical that killed all the salvinia and sunk it to the bottom to decompose. It will do for now. I still want to drain eventually and re-do the pond, but our house will be done in about a month and now that the salvinia is gone and isn't such an eye sore, the pond renovation just got pushed lower on the to-do list.