Anybody have one?

What depth is recommended ?

4 foot ?

I think your guess is in the zone but
test drive different depths doing the type of things you'll be doing every day,
e.g. flip turns, aerobics, volleyball, water polo, etc.

http://rb.gy/l17zp

http://rb.gy/3px1y

IMHO, the SOP 40ft length is only a stroke stroke turn "exercise" haha
25yds clear is the minimum for laps, workouts, preparing to be on a team

Consider variations in bottom depths then research why this might be preferred by the lap swimmers.