I too would be inclined to fence off an area with something or only take them out there on long lines - just incase.
Puppies do go through a phase of no fear, if they choose to just dive in when theres a strong current it could end in disaster.
Its not so much that they are good swimmers, as a pup they will tire very quickly against a curent and wouldnt have the strength to get back on land.
I have one springer that loves swimming - but even at 2 years, we've watched him tire in the river trying to catch up with a swan and it came very close to us jumping in to get him
my other springer loves dipping in and out of the rivers edge but isnt brave enough to swim