So I was wondering: I've fallen through the ice before when walking on lakes in the game. But the other day I was walking along Jackrabbit Island headed towards the next island over. And it was near dusk and I stepped off the island onto what I thought was ice and was instantly immersed in water. There were no weak ice warnings or ice breakage.

It seems like I had just stepped into open water. This was in Costal Highway ( a map I haven't fully explored), so is that normal? Has there always been open water in the game and I just never noticed it before or is this something new?

That's awesome. I'd never come across it before, but I recently got a lot better at the game and managed to last over 40 days. I guess I wasn't surviving long enough to notice it before.

I can totally understand. I haven't been playing much the past months. In fact I still haven't seen the new water in game yet (only video's).

I've searched it for you and this feature was added in Sept. 22 2016 as you can read here: http://hinterlandgames.com/the-long-dark/update-archive/
There's also a video about it: https://www.youtube.com/watch?v=OTx2Fv1dJoU

This open water system also introduced a beachcombing feature. Sounds really nice. :)