It should be safe to still eat the fish, especially if you're going to be cooking it! IMO parasites like the isopod that you found are far less dangerous than less visible ones such as worms, flukes etc.
Quoting from: https://en.wikipedia.org/wiki/Cymothoa_exigua
C. exigua is not believed to be harmful to humans, except it may bite
if separated from its host and handled.
In Puerto Rico, C. exigua was the leading subject of a lawsuit against
a large supermarket chain; itis found in snappers from the Eastern
Pacific, which are shipped worldwide for commercial consumption. The
customer in the lawsuit claimed to have been poisoned by eating an
isopod cooked inside a snapper. The case, however, was dropped on the
grounds that isopods are not poisonous to humans and some are even
consumed as part of a regular diet.
It seems to me that a closely analogous example to the isopod you found on fish are pea crabs in shellfish: https://en.wikipedia.org/wiki/Pea_crab. Though perhaps pea crabs are less parasitic to shellfish than isopods are to fish.
There was once I was having some cooked clams at a resto. When the clams were served, I found every single one occupied by at least one pea crab. They were cooked along with the clams. I just removed and discarded the crabs, the flesh of the clams was completely fine.