The BBC rarely announce airdates for dramas until very close to them actually hitting the screens, so it's a while before we're likely to know anything for sure but it's expected as early Spring.
The BBC are said to be really impressed with Redwater so I'm sure they'll schedule it at the time they think will give it the greatest chance of success.
I know some people will see early Spring as a "delay" but it really isn't. Redwater didn't start filming until April and didn't finish until late July. Then it would have spent months in the post-production suite (with a top editor who has worked on films including Slumdog Millionaire and quality drama including Peaky Blinders).
For example, another BBC drama Apple Tree Yard starring Emily Watson began filming in Jan 2016, finished in March and won't be shown until early 2017. That's just the standard time frame between production and being televised.
