I grew up in Scotland, and apart from the infamous Scottish midges I was never exposed to anything dangerous or poisonous.
I moved to the Middle East in the mid-80s and started diving, mainly in the Red Sea off the Saudi coast. Rarely saw anything that was I many way dangerous, sharks were distant and the most poisonous creatures were really scorpionfish and stonefish, but they don’t really attack despite their nasty reputation.
Then I discovered the Titan Triggerfish, Balistoides viridescens, and quickly learned to stay clear of them when one of them launched itself at one of my buddies, who took a hit in the chest leaving a nasty bruise.
But the most aggressive fish I’ve had a few nips from is the common anemone fish (aka Nemo’s Red Sea cousin Amphiprion bicintus) For such a small fish I advise wearing gloves when getting close to their locale to photograph them.