Serious Sam 3: Jewel of the Nile: Gathering of the Gods - All Secrets 8/8

At the starting area, near the staircase, you'll spot a bunch of crates with planks behind them. Destroy them and proceed through the cavern to discover a Spider Cave, but watch out for a swarm of miniature Hatchling Spiders.
When you climb up the staircase from #1, you'll see a building on your left and a building to your right with a shelter next to it. Go behind the building on the right, jump up on the crates and onto its roof. From its roof, jump on the shelter's roof, and from there, sprint-jump to the roof of the building on the left to collect a secret +50 health.
On the left side of the island, you'll see a floating red object near the shore. Dive into the water below it to collect a rocket launcher.
Near the start, when you encounter the first Cave Demon, jump and climb on the wall, and then travel over it to collect a secret health.
On the right hand side of the Island, before the first temple is a pile of rocks. Shoot one of the rocks to reveal a cavern leading under the island.
Still on the right side of the island, near the opposite side from the start and behind a temple, walk along the shoreline until you find some rocks. Approach the rocks to discover the "Wilson Secret".
On the right side of the island, you'll see a smaller island to your east. Swim to and walk over it to find a secret cannon, go behind it and press the Use button to trigger a humorous cutscene.
At the temple on the opposite side of the starting area in the island, look up to the fouth column (the one next to a broken section of the wall). You'll spot some rocket supplies there. Use the Mutilator or the Rocket Launcher to bring down the column and collect the ammo.