Most teas are safe to consume i would try to stay away from the ones with caffeine. Tea will dehydrate you so d4ink plenty of water too :)
There are teas without caffeine, mainly herbal. Rooibos is great and has added benefits. Other herbal teas can help with different issues like ginger for nausea, but check which ones are pregnancy safe. Pregnancy teas... Some controversy but should be fine in the third trimester.
Decaffeinated teas still have caffeine.
No, tea does not dehydrate you, that's a myth. It's the same thing if you drank a cup of water instead of tea you'll urinate the same amount. Water is water and tea just flavors and adds some nutrients depending on the tea.
All I drink is tea and they do make decaffeinated ice tea if that's what you mean. Now that I'm pregnant again that's what I drink