I sonar search retention ponds up and down Florida's Interstate. Most are 4-8ft with some over 10ft. If they are 3' or less then they will have thick vegetation.And sometimes dead vegetation makes them soggy and you sink down. But sometimes in Florida they are sandy bottoms. Regardless, that small retainer pond probably isn't more than 2 or 3 feet deep.
This is the pond where Gio was found taken from Street View last Nov. which was very dry, no precipitation. I would estimate the depth in the center 2' to 3' at this time. The concrete overflow at the back is an indicator of how high it can get. Probably 6ft to 7ft in the center when full.
In this google earth photo from a couple years ago you can see the same pond at a much higher level. but still not at capacity.
In the last few days of July the Bradenton had 3" of rainfall. Gio went missing on the 1st. A few days later Bradenton had 4" of rainfall over 2 days. I would say that the pond was at least 6ft or more during this time and could easily hide a submerged body until it floated.