Now that my new ROS project (BlueSub-1) is operational ( Low cost BlueSub1 project - Build / DIY ROVs - Blue Robotics Community Forums) I thought I would point out components that significantly reduced costs but maintained operational requirements and reasonable quality.
WTE: 4" PVC pipe, PVC inside connector, aluminum cap plate, 3/16" O-ring, simple strongback, and BR dome cap. About $70. I firmly believe this will go to 100 feet, or more.
Tether: 100 ft Cat5E stranded cable with 3/4" foam balls for flotation. $40.
Thrusters: SeaFlow 750GPH bilge pumps. $18 x 4.
Everything else is off-the-shelf from readily available sources. Total cost came to about$600.
By only using the ROS in fresh water and limiting depth to around 50 feet, I expect a summer of fun exploring Kansas and Missouri lakes. The biggest problem will be finding decent visibility. Summer algae is always a problem. Feel free to ask for other component details.