You can get bowls for fuel water separators that have a threaded port for a water sensor. The sensor connects to the engine harness, for Mercury at least, not sure about others. You get an alarm when there's water in the bowl, before it's so much water that the engines gets any.
I had a plastic secondary tank that got a bunch of water in it a while back. Had to drain the racor every 10-15 minutes, but never got any water in the engine. From my perspective a fuel water separator without an alarm isn't worth a lot unless it's only ever going to see very small amounts of water. You would need to check it far more regularly than anyone I know does and as described above, some times you can't tell easily if it's full of water or fuel.
I had a plastic secondary tank that got a bunch of water in it a while back. Had to drain the racor every 10-15 minutes, but never got any water in the engine. From my perspective a fuel water separator without an alarm isn't worth a lot unless it's only ever going to see very small amounts of water. You would need to check it far more regularly than anyone I know does and as described above, some times you can't tell easily if it's full of water or fuel.