Here, stand on the steps and look up. Shoot a rope arrow into the iron weight to pull the electricity wire out of the water, if only briefly. Needless to say, quickly jump to the other side and wade the last part - there's certainly enough time for this.
Walk a bit over the walkway; from here, pull the raft in your direction, and close to the electrical device in the water. Position it in such a way that it is directly next to it. Now pull the iron weight down, and the electrical device should hit the raft. If not, quickly pull it slightly into that direction from the steps.
Flooded Vault Tomb solution.
With the coast clear, literally, wade through the water to reach the stairs in the corner of the area, which lead right up to the treasury. [1250 XP], [SALVAGE (250)] and the [SHIPWRECK BEACH TREASURE MAP] are all yours now. You can slide immediately back to the camp via the rope.
• You can find a [GPS CACHE] east of the tomb, near a small waterfall and pond, underneath a rock viaduct. You can push the buoy to the other platform with a climbable rock wall to reach it more easily (it can already be accessed via other routes, but this adds one to the list).
• There's some [SALVAGE] east from where you found the GPS cache. It's near a barrel.
• Move to the right/south to find a platform with a [#5 CAIRN RAIDER] next to the tree.
Continue over the wooden walkway to the east, then (somewhat carefully) jump on the pole, making your way to the end. Shoot a rope arrow in the craggy rocks to proceed to the next platform. There's some [SALVAGE] on top of the small ruined building, and inside is [DOCUMENT - #8 ANCIENT SCROLLS (HOSHI: IN HER IMAGE)]. Note the metal barricade to your right, then head through the tunnel to reach the next area.
Head upstairs to find some [SHOTGUN SHELLS] and press the button. Head back downstairs and press the button to open the doors. There's a raft to your left, but it's tied tight with some rope. Walk up and burn it, then move to the generator room on the other side and pick up the [SHOTGUN SHELLS] along the way. Blast the barricade and pull the raft towards you, then hop on to reach the other side of the wall.