Water pump interrupts A/C
Your A/C already has a 24V control line going out to the A/C head. You don't want to mess with that, because the furnace has some logic in it to prevent something called a short cycle, which damages the A/C. You'd be better off interrupting the 24V control line from the thermostat, which "calls for cool". It is on the Y terminal and is usually Yellow.
So you need to find your pump's pressure actuation switch. If it's at an inconvenient location, this won't work. Otherwise find the two 240V wires that run down to the pump proper, and attach a small 24V thermostat transformer ($12) so its 240V wires are wired in parallel to the pump. This transformer must inherently separate the 240V wiring from the 24V wiring, so select one that goes into a 1/2" knockout, or replaces a junction box cover, with all the hot bits on appropriate sides.
Then you run some /2 thermostat cable from there, to anywhere you can intercept the thermostat's "Y" wire. You use this /2 to power the coil of a 24VAC relay. You put the thermostat "Y" wire across the NC contacts of the relay. The pressure switch's "call for water" energizes the transformer, canceling the thermostat's "call for A/C".
Since the relay is low-voltage, we don't need to deal with the firm rules for handling mains voltage.
A/C interrupts water pump
There is a /2 thermostat cable heading out to the A/C condenser unit outside. When the big condenser motor is running, those two wires will have 24VAC on them. Do I need to tell you what to do next? Yes.
Mains electricity must stay inside proper junction boxes and conduit. Low voltage must stay outside. The two can't meet; you can't snake a 24V cable inside a box or conduit containing mains power. As a result, they make relays and such which "sit on the border" - such as RiB relays which screw into a 1/2" knockout hole, and put 240V pigtails inside, and provide outside terminals for the 24V. That's the kind of relay you need to shut off the water pump.
You can also use current-sensing strategies, as I discuss here.