Scuba gear can weigh a good 50-60 pounds, and that doesn't hurt them much, but IF there is a limit, it should be a function of strength, stamina, and endurance.
That's because the neoprene suit and the BCD (buoyancy control device, the vest which holds the tank to your body) cause you to float. You actually need to attach 2-6 pounds of lead weights in order to keep your head at water level.
Making people sink in water isn't just "annoying, pointless realism." It's a "balancing factor" which makes the game more fun because there are different styles of play.
For instance, your knight in shining armor will sink like a stone if he tries to run away into a lake, or swim in an underwater cavern. But the lightly armored thief will have no problem.
Also, it should be based strictly on fatigue. If you run out of fatigue while swimming, you sink. So you can wear a full suit of armor, and if you're in good enough condition, you might float for a while.
Also, items should be divided into "less/more dense" than water. So if you are carrying 50lbs of wood, you would actually float compared to 50lbs of rocks.