Underwater shockwaves
You might not know this, but there are various fishing techniques that are deemed illegal. For instance, bottom trawling is banned in many places since they destroy the underwater ecosystem, and using ghost nets will leave microscopic remnants of plastic, which take over 600 years to decompose.
And then there’s the worst of them all: using explosives. The OP was hit by a humungous shockwave from an explosive that scared them out of their mind. They were fine, but their instructor informed them that they had to go to the hospital once when a fishing bomb exploded too close.