There is always possible danger of electrocution using any electrical powered device around/in water.

The way to make it as safe as possible is to have your shop vac plugged into a circuit that is protected with a Ground Fault Circuit Interrupter.(GFCI) This is a MUST!!!

Also for your added protection wear shoes when you operate the vac, never use an electrical tool etc, outside without wearing good dry footwear. Even if the device is described as "Double Insulated", wear dry shoes.
And never touch any bare metal parts when you are operating the machine.
These instructions may seem a bit over the top, but believe me they will help to keep you alive.

Working with any electrically powered tool or machine outside, is only as safe as you take the time and precautions to make it safe.

Best to secure the vac to be sure it cannot fall into the pond as an added precaution.
"Horton'

Thanks Horton! Very good advice and thank you for taking the time to response to my question.

Due to pond design, the only way I can get at all of the bottom is to enter the pond. I have been thinking of vacuuming while in the pond (waders) with a partner on shore stabilizing the pump-out style shop-vac and running the pump. Powered through GFI. I did this a couple of years ago but with very little water.

Is this high risk and to be avoided?
Mike

Mike, as long as the machine that you are using is protected, by being plugged into a properly functioning GFCI and you are not holding onto any metal parts of the machine whilst in the water, you will be as safe as you can possibly be.
Your machine should have a plastic suction wand, not metal.
It should be secured back from the pond to stop it from accidentally falling into the water.

Providing that your GFCI is correctly installed and you are confident that it functions properly, because you have tested it, using the built in test button or with a simple plug in GFCI tester*, you have done all you can do to guarantee your safety.
*The GFCI plug in cicuit testers are sold at hardware stores.

The manufacturers of GFCI devices recommend that their ground fault circuit interrupters, either, wall type receptacles or panel installed breakers, are tested every month to ensure they are in proper working order.

Like any other electronic device of any kind, a GFCI can fail to trip. This could be due to an internal problem that has developed in the trip mechanism or the device has not been wired up correctly.
So that is why the manufacturers cover their puhtooties and recommend the monthly testing of the devices.
It is printed in the installation and after care instructions that come with the GFCI device.

Always play it safe around water and electricity.
"Horton"

Here is a link that might be useful: GFCI installation and testing.

Hi Mike!
I agree with what horton said.

Every spring and fall, I climb in with my full waders and vacuum the bottom of the pond.

My husband stands guard to make sure I can get in and out and
he turns the vacuum off and on.

I have it plugged in to a working GFCI outlet.

The vac is an Oase Pond Vac.

lsst, said,

"My husband stands guard to make sure I can get in and out and
he turns the vacuum off and on".

You did not marry a stupid man!!!! LOL

"Horton"

Horton,
He has the easy part! LOL
He has offered to climb in and vacuum it for me but I am too much of a control freak to let him!