They have life jackets. They are those yellow foam ones. Not sure if it would fit a 20 month old though. I guess it would depend on how big she is.
We use it for our daughter when she was 28 months. You could try calling the resort and asking on the weight restrictions to see if it will fit her.
Otherwise my suggestion would be to bring your own.
Good luck.
Where on LI are you? We are in Huntington.