Martins do want a water source but what we and others did was put out stock tanks filled with water or large bird baths. They don’t need a lake or a river just a source of water and it should be large enough for the number of birds. Several bird baths also worked. I know the town where I grew up and this was started had no body of water for many miles.
You have to remember to keep your garage door closed when they are colonizing. They can build a nest in an afternoon, and then you can’t close the door again until the eggs are hatched and out of the nest. And the following year, the same birds come back and rebuild.
That was our problem when we first moved here. The previous owner used to leave here garage open. But, we love our barn swallows.