Minecraft – Some Minecraft sound effects on Mac are too quiet

If you want to play it safe, try putting all your gear into a box next to the entrance and take what you absolutely need out with you. Generally, it is a good idea to make sure you can easily see outside nearby, if you can't, you may want to spend some time leveling the zones around you if its feasible.

You can also try putting up a tower from within your house so you can take a look around you before heading out. Just remember to put some torches at the top.

Finally, if you don't mind dying a few times, just stash all your gear and go out and trigger them to blow up. After a while, you'll end up with a nice and flat area to work with.

This is caused by the spawning algorithm. On the ocean, there are significantly less areas suitable for spawning. When you build a house over/on the ocean, you provide a large suitable area for them to spawn in, and because it is the only one in the area, everything that would normally be spread out around there instead spawns inside your house.