Minecraft – Can mobs spawn in water

Simple Mob Grinder

The simplest mob grinder you can do is to use gravity to kill off the mobs.

Build a really tall hollow tower out of cobblestones with no windows. Make sure the hollow shaft is 2x2 wide so there is enough space for spiders to fall down as well as any other mobs. Make sure this shaft is built over something solid - stone, not sand, dirt or gravel. The reason this is all dark is so that the mobs will still spawn during the day as well as at night.

At the top of this tower create a large room, again with no windows. Something around 15x15 will do. Make it about 5 blocks high and keep a hole in the floor over the shaft so that you have a big dark room with a shaft in the middle. In theory the mobs will spawn in this big room, wander around and fall down the shaft, splatting on the ground and leaving a load of goodies for you to sweep up. Don't forget to add a door at the bottom of the shaft so you can get in there and grab it all.

Really that should be all you need. You can expand this idea however you fancy; underground grinders, multi-room grinders... however adventurous you want to go. Bear in mind though that Mobs will only spawn when you are near to them, so you should build a house near to your grinder, or at least hang around near it when you want to get stuff.

If you've built it underground, you'd have to lit all caves around, so the mobs won't have any other spawn possibility than inside your grinder. Your best option is to build it as high as possible (remember that on MCPE maximum construction layer is 128).

As an extra recommendation, I advise you build it over the far ocean. Usually, there are some islands near the shore. Choose one and, from its farthest (relative to the shore) boundary, count around 128 blocks towards the far ocean and then start climbing towards the sky as high as possible for your grinder to fit under 128.

With that distance, mobs won't have any other options to spawn but inside your grinder. Remember to stay around (but not too close, as you already did) for them to spawn.