We’ve dreamed about doing some similar things here. It would be a lot of fun!
I definitely think the T200s are the way to go because SCUBA divers have a lot of drag and it takes a lot of thruster to move around. In terms of batteries, you’ll get the best power and endurance for the size from lithium polymer batteries. I would suggest looking at those designed for RC aircraft as a starting point.
In terms of battery size, let’s say you have two T200s (which can run at up to around 275 watts) and you want to be able to run at full power for 10 minutes, which is probably about right for an hour of SCUBA diving. That’s (2 x 275 watts) x (0.17 hours) = 93.5 watt-hours of energy. You’ll need a battery with that capacity. The power capacity of the battery is the voltage times the amp-hour capacity. For example, with a 4-cell (14.8V) lithium polymer battery, 14.8V x 7 amp-hours = 103.6 watt-hours, which is enough.
Here’s a battery that would fit those numbers pretty well:
How are you planning to mount the thrusters? To the tank? Here’s a very cool freediving video for some inspiration:
Keep us posted!