I’ve been using a 45 degree finder since 2008. They do take some getting used to in the sense of adapting from a straight look-through finder.
As Massimo says, they do avoid neck strain and for shooting near the seabed, they are invaluable. As you point out, for shooting up, which I think is key, they are perfect.
They are a bit harder to use in mid-water situations where, to start with, you might find you are hunting for the subject. Not a problem for wide angle - more for fish portraits.
The 45 degree finder has been, for me, arguably my best investment in my gear. I’d never now dive without one. I’ve transferred it from a D300 housing to a D800 and now D500. It cost about €1100 new and I’ve never regretted it for a second .