Calculating environmental effects on monsters' CR?

[Brauron]

I'm running a D&D campaign for the first time in ages. I usually run Call of Cthulhu, so running encounters that are "fair" or "beatable" for PCs is not usually an issue for me.

I have two players who are experienced, but primarily play hack 'n' slash dungeon crawls. My third player is very inexperienced, but knows how to play, and tends to prefer more story-driven adventures (she writes a LOT of fan fiction).

So I'm trying to accommodate their tastes while also creating unique encounters and challenging them. I'm just concerned that some of my encounters are...too challenging.

For example, an upcoming encounter involves the PCs being asked to rescue a child that was kidnapped by Grimlocks. In following the Grimlocks through the underground, they have to cross a flooded cavern. The grimlocks have a raft, the PCs don't.

The water is three feet deep and cold. As I understand it, this will reduce their move speed. It's also pitch dark. This won't inconvenience the Half-Orc Barbarian, who has Darkvision, but the Half-Elf Ranger and Human Cleric will be at a disadvantage unless they light torches or the Cleric has Light memorized, though at this level it will only last 20 minutes.

So they'll be wading through cold water in, at most, dim light (probably). Midway across the flooded cavern, they'll be attacked by a Giant Cave Catfish (stats of a Medium shark, CR 1).

In theory, this is a CR 1 encounter, which should be easy. Will the waist-deep water and resulting reduced movement abilities, plus the dim light (which will give the fish concealment bonuses against being hit, I'm figuring, without hindering it due to its Blindsense) result in the encounter being a greater challenge?

[awa]

definitely looking at the rules for fighting in a marsh (which i feel best approximate this situation) it looks like the shark would have a +8 bonus to ac + 4 reflex (technically that's for creatures who are size small not medium but the shark is swimming so i think it works.) The light is somewhat of a problem but not enough to make it worth a whole cr. In fighting the shark the speed penalty does make the fight harder but not by a lot since they can't really use ranged attacks against it any way so its going to turn into melee combat pretty quickly anyway.

[Brauron]

Thanks awa, I'll definitely take a look at that.

Seems like the real concern for the players will be when NPCs start using intelligent tactics (flanking, spells like Sleep, Daze, etc.).

[Kylarra]

The answer is yes, situational and environmental bonuses will make an encounter more difficult and thus should [ad-hoc] increase the CR. Blindly going by CR is one of the worst things a DM can do.