You enter the area with the pool from the tunnel which is marked A in the screenshot. Do NOT surface here. The frogman will dive into the water and you will miss the 6th Secret. Swim to the right until you reach an underwater wooden platform with two openings. Here you can surface in the corner between them (where the X is).

Once you have got the 5th Secret and pulled the underwater lever that opens a trapdoor somewhere, swim back outside and head underwater to the left, passing in front of the opening you came from. Swim to the end of the channel.

At the end of the channel, climb out of the water and onto the ledge on the right hand side. Then climb on the next ledge. Crawl to the right around the corner to reach the trapdoor you just opened. Crawl backwards to drop in the water, get the Crystal and climb out of the water on the other side (opposite the underwater wooden platform with the two openings).

6th Secret - Crawl to the left around the corner until you reach the rocks, where you can stand.

Go to the end of the passage and turn right.

Pass the hallway with the guard, drop to the metal ledge, kill the frogman and the guard who was with him, dive in the water and claim the Secret.