Silica Pearls are an uncommon resource in Ark that are used for advanced crafting. They are found on the deeper ocean floor or as uncommon drops from Trilobites. When found in the ocean they are harvested by hand.
There are three main ways to find Silica Pearls that I have discovered so far. The first, and most difficult and dangerous, is in the Deeper Ocean. Secondly, you can find Silica Pearls in the shallow river near the Snow Biome in the North-East. Look for the shallow river just inside the biome along the east coast. Thirdly, silica pearls are an uncommon drop from Trilobites.
To harvest from the deeper areas along the coast you'll need to have worked on your Stamina and your Oxygen. Having either a Megalodon or an Ichythsaurus will make this task much easier. The Ichy is much faster in the water and can outrun any threats, which also means you can get down to the depths and back to the surface very quickly. Lazarus Chowder is also a very good idea, as it dramatically reduces your Oxygen usage and allows you to regain Stamina in the water.
An easy way to get Silica Pearls, however, is farming Trilobites. I find these things everywhere near and in water. They seem so common at this time that I can't really say where is best to find them. If you don't seem them on land near a river or ocean beach, check in the water. You don't get many Silica Pearls per Trilobite, and it is an uncommon drop, but I find harvesting these little critters is the fastest and easiest way to get Silica Pearls.
The Deep Ocean, as in the Deepest parts out towards the edge of the map, also have plenty of Silica Pearls. However, this requires a serious investment in tamed Dinos, gear, and recipes if you want to farm without high chances of losing a lot. As far as farming goes, I don't see much point to going out to the Deepest areas of the Ocean. The requirements and risks vs. the rewards are simply not viable. Now, deep water adventures are another story, but we are talking about resource harvesting.