No Legendary actions, no Legendary resistance, no magic resistance, no bonus action, and no reaction abilities.

This thing is a pile of hit points that poses no serious threat to a late Tier 2 and Tier 3 group. It's breath attack is one of the most resisted damage types. It has a terrible passive perception and (compared to say an adult red dragon with its +11 to initiative) a pretty underwhelming initiative score. It also looks like they removed its proficiency to Dex saves that it had in 2014.

This really isn't CR 17. It has too many weaknesses given its poor saving throws to be much of a challenge. Heck, as long as you stay at least 20 feet away from it at all times the only thing it can do is use its breath on you.

It's like they forgot they made an Ancient Dragon Turtle that has Legendary Resistance, Legendary Actions, etc. https://www.dndbeyond.com/monsters/2059693-ancient-dragon-turtle

Quote from DnDDetective >>

No Legendary actions, no Legendary resistance, no magic resistance, no bonus action, and no reaction abilities.

This thing is a pile of hit points that poses no serious threat to a late Tier 2 and Tier 3 group. It's breath attack is one of the most resisted damage types. It has a terrible passive perception and (compared to say an adult red dragon with its +11 to initiative) a pretty underwhelming initiative score. It also looks like they removed its proficiency to Dex saves that it had in 2014.

This really isn't CR 17. It has too many weaknesses given its poor saving throws to be much of a challenge. Heck, as long as you stay at least 20 feet away from it at all times the only thing it can do is use its breath on you.

It's like they forgot they made an Ancient Dragon Turtle that has Legendary Resistance, Legendary Actions, etc. https://www.dndbeyond.com/monsters/2059693-ancient-dragon-turtle

 I'll hazard a guess that the Dragon Turtle's difficulty is based on where you will likely encounter it: in the water. Aquatic monsters are more effective when you consider they will be more mobile than players, suffer no disadvantage underwater, and often get the jump on parties. Plus, I suspect that this particular monster is designed to be a greater danger to sea vessels than their passengers (though still deadly to them as well).

What happens when a creature becomes prone underwater?

Prone in dnd doesnt necessarily mean "laying down" it basically means you have to right yourself. So how I would play it underwater is the hit puts the character in upside down in the water. They have to right themselves first before they can start moving in a direction. 

First of all I love the new artwork, she looks even bigger then before. A real terrifying creature.

For stats, i only compare to 14 as always. She gets 6 more on initiative and has 15 HP more.   Oh and 10 more swim speed.  Lost his dex save thou...

For dmg changes, her breath attack needs a higher save now and deals 4 dmg more.

For multiattacks, the bite and claw option dealt 26 piercing and 32 slashing. The new one deals 3 bites for a total of 69 piercing and 21 fire! 

The tail optiob dealt 26 piercing with bite and 26 bludgeoning and needed a dc 20 save or get pushed away and lay prone.  New one repkaces only 1 atrack for 46 piercing and 14 fire dmg. The tail only does 18 bludgeoning but auto prones. 

Overall I dont know if its a cr 17 worthy monster but i like the changes

I wonder if you could train them

I currently have one that has aligned itself with a marid queen in my campaign, good thing its on the players side ;-)