How would sea travel develop if certain parts of our oceans were filled with monsters?
It takes place on our lovely Earth. No geography changed at all, and you'll see why I wanted to clarify this.
The smallest monsters are 200 meters in length and can travel at 75 knots, while the largest are 800 meters in length but can only travel at 30 knots. Still quite fast though. For all intents and purposes, the monsters could only possibly be killed by a nuke. But of course, Cristopher Columbus wouldn't be able to get his hands on one.
As I mentioned earlier, only some parts of our seas and oceans would be infested with these nightmareish creatures. For whatever reasons ( insert handwavium here ), the sea monsters need to eat lava, so they never stray further than 100 kilometers from any volanic centre ( and they can go from the surface to the bottom of the ocean and vice versa with ease ). Right now, at the top of my head comes the "Ring of Fire" around the Pacific.
They ( the monsters ) are hyper aggresive, attacking any and all humans dumb enough to travel by water in their 100 km radius.
And for anyone whom might ask ( I don't know what you might need to know this for ), the monsters can travel from a radius to another if they overlap.
How would this affect the development of sea travel?
Edit : As pointed out by user535733, submarine volcanoes can go dormant. Since the sea monsters already have handwavium, let's give them some more and say that they can somehow make those volcanoes active again. Because magic.
Edit 2 : Alright, way too broad. Time to change this question up a bit.
Edit 3 : The monsters will always attack a human within their radius. Going in their territory is asking to be gobbled up by them.