Since it only needs to be energized when the engine is running, can I pull what I need off the key switch?
Yes, that's the right idea--to power it off of the "switched" side of the key switch.
The simplest way would be to to just add another ring terminal to the "switched" side of the key switch, leading to the aqualarm panel.
The next level would be to mount a small bus or terminal strip next to the engine panel. Power this bus strip with a lead from the key switch. Then you could wire the aqualarm panel, the blower switch, and any add-on equipment to the new bus panel.
The next level above that might be to add a fused bus strip next to the engine panel and wire your new components to that. Then, every new circuit you add on to the engine panel (via the key switch) is fuse protected so any shorts on your added circuits won't take out the whole engine panel.
Also, for your heat sensor itself, you'll need a (+) source to power the sensor down in the engine compartment. While the instructions say "+ POSITIVE BATTERY VOLTAGE," I think it's silly to hook this up to an "always hot" battery source--it only needs to be powered when the engine panel is powered. Which sounds like it leads you back to your key switch, except that Ericson has already provided a "switched" circuit that runs back to the engine compartment. It's the alternator excitation/fuel pump wire.
So, in the short run, I'd say you could wire the (+) side of the temp sensor to the (+) supply line for the fuel pump. That way, it becomes energized only when the key is on.
But, in the long run (at least on my boat) there was a pre-existing problem with the fuel pump wiring: Rather than running two "key-switched" (+) lines from the engine panel to the engine compartment (one for alternator excitation, and one for the fuel pump), they only ran one. And the one they ran was the purple, 16 ga wire that was originally meant just for the alternator. So, I had the fuel, pump, the Hobbs meter, and alternator excitation all powered by a small 16 ga wire. This is why I mention, in part 2 of my engine panel blog, "running a new 12ga [10ga] wire from the “ACC” bus [engine panel] back to the engine bay."
In the end, I replaced the purple 16 ga wire to the engine compartment with a 10ga wire leading to a fused bus panel. This panel powers the fuel pump, alternator excitation, the Hobbs, and my engine temp/oil pressure sensors, each with its own circuit protection (green circle, below).
