Just shows how out of touch I am. I’m surprised you had to ask. I thought everybody knew about those. But I guess not. Anyhow, they were popular back in "the day". I had one in my boat to feed my V4-75. Still had to carry a couple of 6 gallon tanks to have enough fuel to go anywhere.

They work just like any other tank for use with fuel pump equipped motors.

Back in the fuzzy corners of my memory, I sort of remember there was a law passed that defines what is a portable tank. A 12 gallon tank is pretty dang heavy, over 100 pounds. And they were made with even greater capacity. So it was decreed that anything over a certain capacity had to be permanently installed, meeting all the requirements of permanent tanks.

BTW, that is an aftermarket OMC hose connector. They’re still made today.

EDIT: Reading the ad again, I see it is a 20 gallon tank. Definitely not "portable".

More opinion from me:

If you mount it in the boat it will not be safe, as when you fill it an equivalent amount of saturated gasoline vapor will go into the bilge (windy days excepted?). I don’t know what the regs are but any permanently mounted tank should be set up with proper fill and vent so spills and vapor go overboard. So why not just use 6 gal portable tanks? The OMC ones, if in working order, seem relatively safe.