Perko substitute?
 

That would work for 1 battery. Most using a switch are running 2 or more.

Is it going to rust? Most of the time when you cheap out on electrical parts for the boat, you find out after the winter why you should have paid for the good one when the part you bought last summer is a nasty brown mess that ruined your carpet.

Perko what, battery switch?" Perko is a marine supply manufacturer and makes probably 10K parts for boats, along with a couple of switches.

That is a simple basic ON/OFF battery disconnect. So if thats what you have, then yes, it would be an electrical substitute. Cant comment to its build quality in regards to the marine environment though. Blue Sea and Guest make a quality ON/OFF disconnect. If you have a dual-bank battery switch with positions for example: OFF/1/BOTH/2, then no, the switch in that link will not work in its place. Blue Sea and Guest also make a quality marine dual-bank battery switches.

my recommendation Add a Battery