ORIGINAL: Additional info with correction ....

To extend the operating range & dive depth try the following:

1) Add additional waterproof sealants to the the receiver box & the joints connecting the sub's three motors.

2) Replace the "stock" transmitter antenna with a "rubber duckie" antenna. I used a radioshack replacement antenna from product # 60-4328. The NPA part # A90441. Save the "stock" transmitter antenna.
3) Re-route the receiver antenna to the inside the sub's hull, from it's present conning tower mast location. The sub's receiver antenna should be able to reach all the way to the rear of the sub, just above the "side thruster" motor.
4) Drill a 1/16" hole on the sub's deck ( right in the middle of the two 1/2" slots). Now mount the "stock" transmitter antenna on the sub's rear deck with a 1/8" dia X 1/2" stainless steel nut & bolt.
(Connect the re-routed receiver antenna wire to the new receiver antenna via the 1/8" nut & bolt.)
You now have a modified Wal-mart submarine!

Modified Wal-Mart Sub spec: Operating range 40 to 50 feet (on surface)

........................................ Max Diving depth 6-7 feet ( in swimming pool water)

........................................ Max speed (submerged) with modified TRAXXAS prop #1534 , ... 50-55 feet per min.

........................................ Sub power source: Four AA 1.2 Volt Ni Mh 2300 mAh