Technically speaking you don't need accessories to go snorkeling, however it is recommended that you do consider buying the following as it will make your snorkelling experience more enjoyable.
There are a number of antifog products available, however in my opinion the best is a product called absolutely clear, simply apply to inside of the mask and it will stop your mask fogging.
Mask Strap Wrapper
This is a must have for women with long hair, a mask strap wrapper is simply a section of neoprene that covers the mask strap and stops the strap snagging in your hair.
If you are at all unsure of yourself in the water or want that extra confidence look at buying a snorkeling vest, these can be inflated or deflated whilst snorkeling and provide a bit of extra buoyancy.
Fish ID Slates
These slates contain /Images of fish and can be bought for specific areas (e.g. Red Sea), if you are interested in knowing what you are looking at these are well worth having, luckily they are made of plastic so can be taken into the water.
Not a neccesity whilst diving but certainly should be considered. Whether digital or not its a great way of capturing those great snorkeling moments.