The AWS CWI doesn't cover hydro testing as a subject nor does it cover the actual interpretation of radiographed films to the degree of you being certified to read films.
The procedure for hydrostatic tesing is spelled out in each applicable code or standard. Basically, pressure vessels are hydrostatic tested at 1.3 times the MAWP of the vessel or pipe tested at 1.5 times MAWP.
What exactly does your company produce?
The CWI does not specifically include hydro testing as a function, nor does any other NDE testing method. If you are producing pressure vessels, you do not need any specific "certification" to witness a hydro test. The process and inspection requirements should be spelled out in your companies QC Manual (that has been reviewed and accepted by ASME/NB). You should start by getting a copy of the QC manual and reading it.
Now, with that said, ASME does accept in several of the codes the CWI certification as an alternative to ASNT VT certification. BUT.... again, it goes back to what your company QC manual states. When doing Code work, your QC Manual is your guide.
I do hydro test all the time as we do ASME work and NBIC repair work. I do not have any special "hydro" certification. What I do have is authorization in my company QC manual to do the inspections and a 10 year working relationship with the local A/I. In my opinion, the CWI is a good certification to have though, it does lend credibility to what you are doing. If the company will pay for it, go for it. If they don't pay for it.... that's a big chunk of $$$ to shell out.
