The buildings below the island can not be accessed by vehicles, the workers have to swim to get there

This is Coast Guard Island in Alameda. Once you get the main walls up the zombies will only come over the single bridge, making an excellent kill zone if you are using snipers and assault rifles. Downside is any attempt to do anything off the island requires you to more or less take the same bridge. getting cars for your squads is a must and I've found it helps to secure a foothold on the mainland side of the bridge to shorten the run distance for workers.

Where is this?

can the zombies spawn on the island?

It's practically a fortress.