Which reminds me, any more specific ideas on how shipbuilding and naval warfare might develop in your scenario,
@Bear Ribs?
My general thoughts:
Sea monsters live near the shore in their larval/youthful stage but move into deeper waters as they mature. The deeper the ocean, the more horrific the sea kaijuu get. River, lake, and shore fishing are still generally viable with only a slightly increased chance of giant crocodile/colossal crab attack.
The Depth Charge will get developed very early after the discovery of gunpowder, however the deep sea horrors take more gunpowder than you're likely to bring to the fight and explosions may attract the attention of more monsters so it's not a panacea.
In general, I would say "medium" ships don't exist, boat strategies for dealing with monsters will go two ways: Very small, very fast craft that attempt to outrun sea kaijuu, and the biggest most armed/armored ships you can build that try to fight them. Ships in the middle get overtaken and eaten so nobody builds those.
The first transoceanic trip (That's known to survive) will be an ironclad steamship, such travel won't be reliable and bigger, badder ships will be needed to travel without risk. Airships can safely cross the oceans once developed to a sufficient degree to have the range, and early colonization efforts in the new world will work off of flying machines as ironclads aren't reliable enough.
Trade Routes will develop based on following undersea ridges like the Emperor/Hawaii sea chain, allowing ships to stick with shallower waters and avoid the bigger kaijuu.