Hmm. Clearly, the -intended- rule (and certainly the sensible one) is that a pull/push/slide must be legal movement using targets movement modes.
A swimming creature can be pulled around vertically or horizontally in the water -- but unless it has a fly or walk speed, it can't be forced moved out of the water.
A creature with a fly speed can be pulled/pushed any which way.
A creature with a walk speed but not a fly speed can't be forced moved into the air, but can be moved along a slope.
Hmm. Only thing is, this doesn't cover stuff like critters that force move people into water--though that's arguably hindering terrain anyway. Not seeing a big loss here.
A swimming creature can be pulled around vertically or horizontally in the water -- but unless it has a fly or walk speed, it can't be forced moved out of the water.
A creature with a fly speed can be pulled/pushed any which way.
A creature with a walk speed but not a fly speed can't be forced moved into the air, but can be moved along a slope.
Hmm. Only thing is, this doesn't cover stuff like critters that force move people into water--though that's arguably hindering terrain anyway. Not seeing a big loss here.