They develop the story of The Drownsman early and through-out the film. I really do appreciate the attempt to create a new iconic slasher type villain. There are too many modern slasher rip-offs that don't bother to give their killer any back-story or anything resembling an iconic look.
The problem is in the execution.
As I mentioned, The Drownsman is essentially the Freddy of water. If you are near water, he can appear and strike. COOL! Everyone has to sleep, and everyone has to be near water. So, I was envisioning a film where a character would drink some water, only to be ripped apart from the inside as the water went down their throat. Or maybe someone cries and a tear cuts it's way down her face. Unfortunately, he just uses water to get a hold of you, then pulls you into another dimension where he drowns you so he can hear your heartbeat echo through the water until you die. That means that the tension that should be building for the "kill" is just a transition, and then the actual kill is boring, as the victim just drowns while he listens.
Another problem is that his powers seem to change. In the real world, he is only able to be where water is. However, he is capable of making water appear by the gallons, turning on sinks or making refrigerators leak etc . So, why does he only do that occasionally? I would have preferred it if he had no power over water at all, and instead relied on people naturally being near water in order to make his move. It's kind of like Final Destination...I prefer for the deaths in those films to be insanely improbable accidents, and don't like when it is clear that death is actually "chasing" them. And why is a killer who has power over and can create water afraid of small fires?
Also, there is a plot point that comes out of nowhere and ends up being very important, when they really hadn't built up to it well enough. More time should have been spent on developing Madison's phobia and life, so that what comes later might mean something, rather than rushing straight into it. For instance, she sure does look like she's had a shower recently...so...is her shower somehow inaccessible to The Drownsman (again, it could have showed her getting attacked in the shower, then go the rest of the film without bathing)? The added plot point I mentioned seems like something that could have been saved for a sequel (and in fact is generally something we could expect from a franchise out of ideas but trying to justify another sequel). This film rushes into the second act within minutes of the opening credits, so we miss out on what would be vital world-building. What should be the event that leads to THE climax of the film is actually one of the first scenes in the movie. There is one character who leaves her wedding to have an argument with Madison, then stages an intervention the next day, then spends the rest of the film with Madison dealing with The Drownsman. Her husband is never referenced again, which is fine since they apparently had no plans together for after the wedding.
Other stuff is just stupid. The lead character watches one of her friends die...just stands there watching her drown...and then...once dead...she THEN frees her from the trap.