To unlock the three Swan Diver achievements/trophies, you need to swan dive off the three highest points over water during the course of one run through the game. These screenshots show the locations. If you need more detail, refer to the individual level walkthroughs, or check out this YouTube video by F4LLENS0LDIER.
Just to review, to do a swan dive, start moving forward toward the edge and then press Jump followed immediately by Crouch.
SWAN DIVER I - Mediterranean Sea - Niflheim
After killing the kraken, chimney jump up the alcove on the right to reach the alcove above, where you'll find treasure #15.
Swan dive from the alcove into the pool below to unlock the achievement/trophy. Note that you must take a running jump before diving, in order to reach the deep water; otherwise Lara crashes into the steps below and dies.
SWAN DIVER II - Mediterranean Sea - God of Thunder
After climbing the containers to the upper deck, go to the left, take out the two mercenaries in the hallway and then backtrack to this white bin with the words "Life Jackets" stenciled on it.
Swan dive off the bin into the sea below to unlock the achievement/trophy. Then swim over to Lara's boat, jump back to the anchor chain and climb back onto the deck. (Or save the game and reload at the previous checkpoint.)
MASTER SWAN DIVER - Coastal Thailand - The Remnants
This swan dive location is just before the spot where you jump down to the wide rock ledge to get treasure #4 and just after climbing up between the two walls, shown above. Before diving, you may want to continue up the path a little ways to the next checkpoint and save your game.
Then return to this spot and take the swan dive to unlock the third achievement/trophy. If you saved at the checkpoint, you can reload and go for treasure #4, instead of having to climb all the way back up.
[Rewards | Niflheim | God of Thunder | The Remnants]