Having done a guided dive or two to 100 ft is not the same as just diving to 100ft and may not give a full experience. Most of the decision making on a guided dive is taken care of. When diving without a guide and with a buddy the decisions are up to you. (Actually with a guide you should be ready to make a decision too of course)
I do a lot of dives to 80ish ft. I do not worry about narcing but I notice a slight slowing if I have problem solving to do. Once I hit 100 ft there is definitely some slowing of my thinking. I find I need to make an effort to keep on task and check air frequently. Sense of time is off. My dives all have a hard bottom at 115 ft or less. Have had one dark narc on a ledge at 105 ft. Was an over cast day so dark and gloomy. Got a very strong feeling of dread. Went away once I went up a few ft and got back within sight of the anchor line. Had dove this ledge before without such an issue. I do not do instabuddies past 90 ft. Dive either with an experienced dive buddy I know or with a hired DM.
@Steve_C
Totally agreed on the non-guided dives. And once you’ve felt that dark narc, you will never forget it. I’ve not felt it since the 117ft dive last year. Perhaps it’s because I’ve got more deep experience now, but I don’t know. My dives are all hard bottom, but depth varies.