A tub really won't be a fair test, although it would indicate a manufacturing problem off the bat.
When you use a detector, you swing it, bump it, jostle it, expose it to direct sunlight, shade, breezes, changing temperatures, pressure on the display plate and button panels as you press the buttons, and actual water if you use it in the water for searching. Any of those things could expose 'seal' problems, or even cause them. If you soak the thing for 24 hours or whatever in the tub and then go out use it in water and it craps out because of any of the above combination of factors, you'll probably be that much more upset.
In addition, if there is a seal problem, it very well could have been caused by being soaked in the tub, but if it shows up later, you won't know what the cause was anyway and you'll still have to send it back.
Go swing the thing and find some treasure, in my opinion!
- Muddyshoes