RStringer....personally I wouldn't use them in the pond, but it's probably not the end of the world if you do. I see people use them as retaining walls at some lakes or at boat launches. I have a few around my place just not touching the water.

Water Contamination Dangers
Railroad ties can leach creosote into soil and water systems. Eating food or drinking water with high levels of creosote may cause burning in the mouth and throat, stomach pains, severe skin irritation, convulsions, and kidney and liver problems in humans. If the creosote leaches into the groundwater, it can adversely affect the ecosystem and the water quality. Sealing creosote-treated wood may help a little, but the chemicals may still find their way into the soil and groundwater. Railroad ties that are oozing creosote or smell strongly aren't safe to use.

Dangers of Railroad Ties

Fishing has never been about the fish....