It's going to get harder and harder to do that as cellphones get better though.

iPhones already have satellite SOS feature which works worldwide, and are starting to roll out satellite texting for non-emergency use. There are a few Android models that are slated to do the same, and it's only a matter of time before most phones can do this.

There are plenty of phones that are waterproof (or rated for submersion in 5 meters of water for 30 minutes or whatever) and that's only going to become more common too.

My phone lasts for about 2 days on a charge with how much I use it, and I charge it every night. That's only going to get better with better battery technologies (the trend of phones getting thinner in response to increased battery capacity has actually somewhat reversed in recent years).

So, in a classic horror movie scenario with 5 or so people they'd need a reason why every single person is out of charge or has their phone broken. Even if the protagonists can't get themselves out of the situation they're in using their phones (because they're broken or whatever) you still need to answer how they got into that situation in the first place if they have offline maps and GPS navigation. That's not as big of a problem but it eliminates "they got lost" as a premise for why they're in some spooky woods or wherever.

It seems to me that you'd either need to set the story in an abandoned mine or make the antagonist explicitly supernatural.