Dave,
I would suggest putting some mesh in the bottom of your scoop.
This is the scoop that I use and I very seldom have targets fall thru. http://www.findmall.com/read.php?26,1470505,1471756#msg-1471756
I also use a vibra probe, but I use it to find targets in the bottom of my sifter after I have retreived them from the bottom. Makes finding even the targets that are very small pretty easy.
http://www.findmall.com/read.php?26,1475832,1476394#msg-1476394
The heavier wetsuit will help, but it does have to fill up with water so the tighter it fits the better.
I would suggest staying consistent in your depth that way the water won't have to drain out and then be replaced with some more cold water.
The last thing I would like to mention is using a weight belt can be very dangerous, if you don't follow all of the safety rules associated with that use.
The weight belt should be a quick release model and you should be very familiar with its use. So you get it off in one motion while freaking out because you just stepped from 3 ft of water to 6 ft of
water! Its the last thing on always so that it can be the first thing off if needed.
I'm not preaching to you, just want you to be safe and have many more hunts!
Steve