If a pool is in a room and it divides some 5 foot squares diagonally in half ...

[Mir]

If a pool is in a room and it divides some 5 foot squares diagonally in half and a PC enters the square are they now in the pool (5 feet deep)?

What if they are dragged into that square by a large creature with reach doing a grapple with a tentacle?

-tempted to just change the pool shape.

Given that same large creature lurking in the pool (the pool is 10 by 10 with additional points on opposing ends which split the squares, 5 feet deep) .. Can it do a reach attack to someone 10 feet from the pool or only attack those next to the pool? Is it different if it is standing or lurking beneath the surface?

[Bizbag]

The rules (and comments by devs confirm) deliberatly avoid dealing with 3D movement because they get messy fast. Just keep it simple. Does it have 10' reach? Is the target 10' away? Good, you're gold.

If you must, adjudicate that if it's lurking beneath the surface, it's 5' "down", and thus can hit next to the pool, and only be hit by reach weapons from the pool edges.

Try not to get crazy with angles, and triangular distances, and square root 2s and all that. It's really not worth it.

If a character's dragged into a square that's half-and-half? Have the tentacle monster make a Grapple check to drag him into the water (assuming that's what he wants to do). If he wins, the dragged player enters the water (in the same square).

[Starglim]

An "edge of the pool" square has a solid surface 2 1/2 feet wide that is probably slightly slippery, and a water area 2 1/2 feet wide.

The solid area doesn't require Acrobatics checks to move through or occupy. Since a person in it is not using Acrobatics, he is not flat-footed.

A creature in the water is probably not squeezing unless, for some reason, it doesn't want to stick any part of its body out of water. See the consequences for 5-foot-deep water in the Environment chapter (Marshes, IIRC).

A creature therefore can be either on land or in the water and usually takes no other consequences for occupying that square. A creature that drags its opponent into the square might have the option to put the opponent on land or in water.

If a creature is lurking under the water, the surface counts as improved cover, both ways, for Perception checks and ranged attacks. If it's Large and hiding in 5 feet of water, it is probably squeezing, prone or both. If a Large creature stands partly out of the water, it has a height of 5 feet above the surface and 5 feet below, rather than the 10 feet height it would have if standing on the land, but otherwise can attack to its normal reach.

[Mir]

I hadn't noticed the information under marsh but I had found the information about something attacking from water and AC.

I think I'm probably going to houserule some of that because I find it a little to much of a bonus for whatever is in the water. Most of this I'm finding my intuition is right on but I really want to make sure I know the rules as they are to start with. It can be really easy to miss something.