Here Be Dragons
For one reason or another (maybe not having an ice age), the sea life is... large. Very large. Whales are the size of large boats and armored. Their predators (think Mosasaurus from Jurassic World) have beaks capable of crushing that armor. They roam all coastal seas and boats are bitget bitten in half before the Mosasaurus discovers there are only tiny snacks inside.
So humans use boats on rivers all the time, but get anywhere near the sea and it's just too dangerous. Every generation some foolhardy adventurers take a boat out to open sea and get themselves killed in front of the townsfolk just to remind the rest that it's still a really bad idea.
The aquatic predators can be of any type you like of course, but since they're still in the middle of an evolutionary arms race with the other huge sea life, they should be huge and mostly immune to anything humans can come up with.