This will cause the mech and submarine health to not change.
The health for both will not increase or decrease while the cheat is active.
If you want your mech health to recharge when getting to a save point or health kit, you have to turn off the cheat temporarily.
Just re-enable the cheat after your health refills.
I do not know how to create invincibility for the cat.
It doesn't seem to interfere in any way with collecting any of the upgrades when active.
But use at your own risk.
Update: Added a version 2 in an attempt to fix an issue with the submarine health not always activating.