There are sharks in the water and some of the islands are home to things that will kill you...but good news, your swimming skill will go up if you swim for it!
You need to make it back to the boat because you can't zone out of OOT without it. Since you're on the FP side, try to swim for
Zachariah Reigh island (#2 on the map) and wait at the dock for the boat to pick you up. (Make sure it's going the right way!)
If you're really lost, ask for help in ooc.