I second the tum tum, LO has been using them for water since 9/10 months x
We use tommee tippee sippy cup's.
Is it they are struggling with the straw? With my little girl I used a fruit pouch to give her the taste and then squeezed a time bit onto the straw. She then started to suck and realised water came out, she’s been fine with it ever since!
Tum tum free flow straw cups, I swear by them