You need to be very careful about using any kind of termiticide around a body of water that contains fish or other aquatic life since most termiticides could harm the fish. Boracare can be used on the raw wood, then once the Boracare is dry, you would want to use a water sealant to protect the treatment. Be sure that you apply the Boracare away from the water so there is no runoff, overspray, etc. into the water.
You need to be very careful about using any kind of termiticide around a body of water that contains fish or other aquatic life since most termiticides could harm the fish. If you are removing the infested pieces of wood, then you may not need any other treatment for active termites. Something you can do is to use a wood treatment on the new wood and any other raw wood on the bridge and/or the pergola. Boracare can be used on raw wood, then once the Boracare is dry, you would want to use a water sealant to protect the treatment. Be sure that you apply the Boracare away from the water so there is no runoff, overspray, etc. into the water.