Is it possible to submerge transducers into a large mixing vessel?

Yes, this is possible. All transducers are watertight, and can be temporarily submerged or installed into vessel as stable measurement installation.

What is maximum applicable RF voltage on transducer?

During operation, the maximum RF voltage on transducer is 150 Vpp, but the transducer can withstand 200 Vpp.

What is maximum transducer cable length?

Transducer cable carries small signals during receiving phase, and it is not advantageous to use superfluous length of cable in environment with strong electromagnetic interference (EMI). Without any specification transducer cable length is 4 m, but the shorter the better. You can get cable length up to 10 m for the asking when ordering, but for lengths above 4 m Met-Flow cannot unconditionally guarantee transducer performance in environments with strong EMI.
Less problems are met with low transducer frequencies while one has to be careful with higher frequencies (4 and 8 MHz).
When considering the relay box configuration the previous applies obviously to the total cable length from UVP instrument to transducer.

Is it possible to extend or shorten transducer cable?

Cable is integrally molded into transducer, and this is why it is not possible to extend the cable. But you can shorten the cable from the BNC connector end.