Dived with 12 lbs today - and was absolutely fine. I think 12 (or even less) + steel tank, no hood/vest is a good combo. 18 is way too much.
Perfect conditions for shark point today - awesome weather, flat seas. Easy entry off the point, no surge to speak of at the bottom, vis wasn't great but was still a good 10m. Most interesting bit was the boulder fields, lots of life there (schooling yellowtail and bullseyes, few groupers). Surfaced way north of the entry, but managed to clamber up the rocks anyway. Good fun.