I personally would not trust any FDM part underwater. I would only be trusting an SLA part, which once you get your part designed, you should be able to get printed at most any print house.
Fusion 360 is probably the simplest free modeling tool, and with youtube and some calipers, you might be able to model both connections.
I'd go back button focus with focus peeking, and rock the camera back and forth to get the DOF where you want it.
UW MF for super marcro sounds overly complicated.