How do Minecraft mobs behave in water?

Question

Several questions here related to mobs and water:

1. When mobs enter water, do they swim at the top, or will they dive down?
2. Can mobs spawn in the water?
3. I assume mobs can exit water one block deep, but can they climb out from a deeper sheer edge? (Players can, right?)
4. Is there an easy way to make a shoreline mob-proof (without actually using a wall or fence?)

Thanks for the help.

Answer 1

1. Mobs will always swim to the surface, even if you're submerged beneath them (with the exception of Squids, of course). Note, however, that Skeleton arrows can and will pierce water.

2. Mobs cannot spawn in or on water (again, with the exception of Squids).

3. Since mobs swim to the surface of standing water, they will always be able to get to the top of a sheer edge, then exit from there. They can't swim up waterfalls, though, so you can keep them down using running water.

4. A long Soul Sand gate can be built along the shoreline, which will prevent mobs from crossing it. These gates can be traversed by players using minecarts on tracks placed over the Soul Sand. From the wiki:

When placed on the ground in front of a two-block high doorway, soul sand prevents players from going through it from that direction, making it useful for building one-way passages or invisible barriers. This affects mobs too as they have to jump to get out of the slowing effects of the block, thus are unable to get through the door.

Because minecart tracks can be laid on Soul Sand like most other block materials, this quirk can be exploited to create minecart-only gates (by placing soul sand on both sides of the doorway). This is one of the few ways to allow minecart tracks to go directly inside a secure area without risking wandering mobs getting in.

Here's a snapshot of a small one I built beside my house, with a crossing point for players.

Answer 2

To answer the first question:

When mobs enter water they swim to the surface. My son has built a mob "generator" which relies on this principle to draw the mobs to the surface where they die and deposit their goodies for him to collect.

Answer 3

Eventually, Mobs will die if in water too long. Squids will not die, but they are not mobs and will not harm you. (Note: Squids can't swim upwards. I hope this is useful in case you ever find a squid farm!)