Not to the extent that Poltergeist is lift from Jaws
I’ll fully admit I have not seen Poltergeist in a really long time and I have never been able to sit through Jaws (here we go…), but I’ve never heard that before.
If you get a break I’d recommend both. They are both good films and worth a watch the plot similarities aren’t as annoying as the two films aren’t meant to exist in the same universe and they aren’t as extreme as the situation here. I’m surprised you haven’t made it through Jaws.
You could chuck in CE3K into the trilogy and that would almost have been of relevance as Poltergeist and Gremlins both came from the aborted sequel to CE3K which also became ET.