Standing in the Caribbean sea?

I had seen a travel show where people could take a guided boat trip out somewhere in the middle of the Caribbean and stand in 4ft. water! It was a very shallow spot but it wasn't an island as it was completely submerged in water and you can see nothing but ocean as far as the eye can see all around you. I think they said you can only do this at a certain time of year and time of day in certain spots. Is this true? Does anyone know where i can find out more information about this? any websites or trips i could plan? anything would help? Thanks!