To unclog a shower that has standing water you should try to remove the standing water first.
If you have a wet/dry shop vac you can use the shop vac to suck up the standing water.
Then pour some baking soda and vinegar down the shower drain and allow it to sit for around 20 minutes to 30 minutes.
Then get some boiling hot water and after 30 minutes or so pour the boiling hot water down the shower drain and if the clog is not too severe then the drain should clear.
If not you may need to use some Drano and allow it to work in the drain for a few hours and then pour some boiling hot water down the drain again.
If that does not work the n you may need to find the clean out and snake the drain or remove the screen from the shower drain and run a plumbers snake down the shower drain to clean the clog out.