The Sea Glitch is where the goat sinks to the sea floor, you can run around and even into rocks there.
There are many ways to make this occur,
1. Lick the boat by the hotel and wait until he goes past the barrier, try and jump past the barrier then you should sink to the bottom
2. Double jump past the barrier, this doesn't always work though.
To fix the Glitch
If you go to where the island would be under the water then turn around until you see a skyscraper, keep going towards the skyscraper and eventually you will find a yellow-sh block, go under the block and jump continuesly then walk backwards then you should pop back up to the surface