Is fish tailing a problem for Grand Marquis?

I think you need new tires. Get ones that have a good wet traction rating.

Can also be caused by alignment issues or a bad brake proportioning valve or bad suspension components but most commonly caused by worn tires like FoR notes.

Need 4new tires//keep off the gas/you wanna go /go slow not fast

Rear wheel drive cars can handle like that in wet conditions, especially if the tires are worn or something suspension related is broken at the rear of the car. Get it looked at by an alignment shop and make sure the tries are properly inflated.