First lets start with what is a beach. A beach includes the sandy area and first part of the water along the coast of each island. Skeletons are limited to land areas and the beach as they can’t swim. It’s also worth mentioning that row boats spawn on beaches. Normally you’re able to see through the water from your pirates perspective. While you can find some treasure on the beach area, generally speaking, these spawn on the land, more so than the water for example messages in a bottle, trinkets that you can turn in to the Gold Horders, animal cages for the Trading Company and so forth. Treasure maps often have treasure located around the beaches however, the area that wets the sand until your swimming, is generally left empty. So the beach reaches from the sand, to as far as you can go, without starting the swim animation.
The shallow area is the small area between the beach and deep water, specifically, where the swim animation starts and where the cliff drops off under the water. Sloops can generally sail in most shallow waters and a brig if you’re feeling brave however, galleons shouldn’t access the shallow waters due to the hull bottoming out on the land causing damage to the ship. In order to see in the shallows, you need to submerge yourself under water. Quite often you’ll find treasures sparkling on the sand in the shallows as well as mermaids. The waves in the shallows are also somewhat subdued and small sharks can swim in shallow water.
Deep water, also known as the open ocean is where when you reach a darker shade of water and waves are often visible. Deep water would probably drown a pirate before you reached the bottom and to the best of my knowledge, there isn’t any reason to swim there. Sunken ships and supply barrels also appear in deep water as do little sharks. Galleons can sail freely about the ocean and this is also where you’ll encounter skeleton ships, krakens and megalodons.