So any tips on how you would write this. It's an interrogation scene but the Mc is woken by a cold splash of water in a dark room. I don't think water has an onomatopoeia so I can't use that. Any suggestions... And I did search but couldn't find any related information online... Other than screen written things (don't count).
Maybe start of with the sensation the character feels as the water splashes their skin. Better yet, do the icebucket challenge on yourself, that way you will have personal experience to draw from! Perhaps he also gets some water in the wrong pipe which makes him cough? There is plenty to toy around with!
I'd certainly wake up immediately in the flight or fight response. I once woke up in a room with "complete" darkness but had fallen asleep with some light (it was a cruise ship and they had turned off the hall light that normally came through at the bottom of the door). My eyes popped open and I startled. Shooting up in bed kicking and yelling for help. For some reason, my sleepy mind immediately latched on to the idea that I had been put in a box. So I'm yelling, increasingly louder, that I'm in a box and trying to kick and punch my way through. Until the other people in the room informed me what was happening and I immediately sobered from my half asleep induced craze. So, the way I'd go about it as waking up startled and trying to swim. Immediately thinking I've been thrown in a cold river. Once the water cleared from my eyes I'd just look around shocked until all the pieces clicked into place.
If a person is dreaming when something happens, there's a tendency for it to be incorporated into the dream for a moment before they awake, even if it's a sudden shock. Once for instance my dog jumped on my bed and I dreamed a werewolf had attacked me. This actually fits right in with what Marshipan said. You could begin the scene with a brief dream segment that ends with a sudden plunge into water. Maybe something very pleasant like lying in the sand at a beach soaking up sun, when suddenly Ker-Sploosh! (yeah, maybe don't go for the onomatopoeia...). An approach like this allows for that reversal of fortune––a sudden switch from extreme pleasantness to the opposite, and then the MC wakes into the new reality.
You could always get someone to throw some cold water in your face while you're asleep and see what it feels like.
