There are only four coins.
From the starting point:
-Straight ahead, climb the small ledge onto the ruins. Navigate around them (whipping, jumping and wall-hugging) until you reach some stairs. At the top, jump over the gap and fight a swordsmen. Around the corner, fight another swordsmen, then retrieve the coin from in front of the collapsed wall.
-Back at the small ledge, swim toward the underwater ruins. There is a shark circling and some speargun ammo on the ruins. Grab the ammo, then swim up the sunken stairwell. At the top, proceed until you reach a small ledge facing out on a temple entrance. You need to jump at an angle towards the sloping wall and land on the temple. There's a crack in the wall of the temple. Find the demo charge- a coin is within.
-To get the demo charge, walk up the fallen pillar, then cross over to the ledge with a burning fire. Grab two demo charges, jump in the water and swim to a cracked wall (back in front of the temple). Beware the shark, blow the wall, grab the coin inside.
-Return to the temple by climbing out of the water and repeating the angle jump to the temple ledge. Collect the coin inside, then return to the ledge with the burning fire. Grab the third demo charge, then proceed until you find the next cracked wall. Inside is a fountain.
-Continue on until you reach a dirt pathway that leads to the tiled floor of a long hall. There is a large gap in the floor, but before you jump it, drop down into the water. There is a shark, and the final coin rests on the floor by a light.
-Climb out, jump the gap, then put the coins in their respective slots at the end of the hall.