Always happens:
When swimming and reaching shore, my character disappears through the landmass until only his backpack is still visible. After a second or so he will pop back up, clamber up from crawling position and, while doing that, slide (on his belly) further up the slope as an eel before getting back to his feet and start running properly again.

Sometimes happens: If the slope is rather steep, character sometimes "falls" through the ocean floor (sometimes even up to his head). This makes the character get stuck for a few seconds before he "pops up" again to the surface. After that you can start swimming again.