You don't gave all information that are needed for such calculations. However, the technical details of the Thrusters give a peak consumption of 350 Watts each. So you have 6 of them, makes overall 2.1kW/h.
Lets assume your Batteries have the following details: 6S 22.2V 3300mAh.
Therefore the amount of energy the Batterie stores is: 22.2V*3.3Ah = 73.26W/h.
Moreover assume that you will drive only 25% full-speed in Average, 2.1kW/h * 0.25 = 525W/h.
Let's calculate the runtime under these assumptions: 73.26W/h / 525W/h = 0.139542857h * 0.6 = 0,083725714 minutes runtime.
So if you want to archive 20minutes operation time reaarange the formula:
525W/h * (1/3h) ((== 20 minutes)) = ~158Wh / 22.2V = ~7.1Ah
So you need a battery which have at least 7.1Ah capacity at 22.2V, assuming an average load of 0.25%.
Please consider, you will have a peak-load of 25A * 6 = 150A. This means the given battery (7100mAh) needs to support a minimum discharge current of 150A/7.1A = 21.12C.
If you don't respect these you battery might begin to burn, causing harm to your vehicle and the Environment".
Moreover you don't gave information about the consumption of any electronics inside the AUV, therefore this are not in the scope of this post.