The OP says:
“The valve is at the deepest part of the pond, about 40 feet deep, and there is a ton of silt in that area”
So,apparently a diver is needed to even operate the valve……..…… Seems like an extraordinary design flaw.
Yeah, more information would be helpful. Perhaps even a sketch of the piping
PS: Those inflatable plumber's test plugs are only rated for about 10psi max......... They are used for low pressure testing of commercial DWV piping
