After you get the trencher boot, you can now beat Riptide Reef without any problems. Although you can manage to swim from beach to beach with it, it is highly recommended for you to avoid it because you may end up being stuck.
Tough as a boot: Break a crate with Trencher Boots
Crimson Collector: Collect Bloodstones from all over the world.
Drown for Swimming: Drown an enemy by getting it into the water.
You can get the heart piece from the treasure chest at the end of the island. It’s hard to miss it.
The first portion before the double dueling statues have two bloodstones before, they are hard to miss.
Again, you should only visit the Riptide Reef after you have obtained the trencher boots from the Frozen Palace. The trencher boots allow you to jump from edge to edge. Without it, you are highly likely to stuck and have a really hard time to swim back out the island.
For the puzzle where you have to two statues where you have to push them into a duel. Push the moveable box from all the way on top of the cliff. Push it down. Note that you will have to time and place your explosives to make it work.
If you are stuck, try to swim to other beaches as it will work.
Place the bomb at the tombstone once you open the door to blow up hidden passages. You will then obtain the Master Key.