The second part of this sequence begins at the stream below the Flooded Vault Challenge Tomb entrance. The route from The Grotto Day Camp to this point is shown here. The route rom the Survivors' Camp to this point is shown here.

Once you're standing in the little stream below the wooden bridge, use the Melee button to shove the rope-wrapped float out into the middle of the stream.

Keep shoving it until it goes over the waterfall and lands in the tide pool below.

You can't drop down the same way, though, or Lara will die. Instead, climb onto the bank and jump over to the climbing wall. Climb to the top.

Ride the zip line down to the top deck of the multi-level shipwreck.

Then ride the next line down to the beach at the base of the tower.

Head down to the left to find the float sitting in the pool.

Push it...

...and/or pull it with a rope arrow to position it below and to the right of the broken pier.

This takes some effort, but eventually you'll get the float in position. Then you can use it to jump up onto the pier and get the GPS cache, plus a little snack from a food cache. Go ahead. You deserve those canned peaches. ;)