Is there anything for you if you return to spawn island?

Yes you can return to spawn Island, No the weapons on the tables will not be there.

If you have a lucky plane ride and are able to drop on the north east coast of the Main Island, and you are able to find a boat, there is usually enough time to ride over to Spawn Island and return before the circle shrinks.

In terms of what sort of loot you can expect to find, all of the "testing weapons" that are placed during the setting up game-phase are removed before loot is spawned so no chance of finding crate only weapons just lying around. The island does have a decent amount of loot (usually enough for Duo's but a full squad will probably find it lacking) with a decent chance of finding military grade gear.^{See Map Below}

Yes, you can return to this island, just take a boat !

There will be some weapons in the underground bunker or in the buildings but none on the outdoors tables. The rare weapons like the AWM can be found in supply drops, not on the starting island.