Is Palkia with hidropump any good?

Not overly, kyogre is far better as a water attacker, Gyarados, Hydrocannon feraligatr also are very powerful water attackers that do better.

I just finished maxing a 100% Palkia with DM (obviously). I can't think of a scenario (in the next 5 years) where I'd even give that perfect one a 2nd charged move.

Short of a change to the type-chart in the main series that's reflected in Go, it's impossible for a 'mon to be weak to the combo of Dragon and HydroPump. If something is weak to dragon, that means that it's Dragon type itself. And since Dragons resist water, the best that you can hope for is weak to dragon/neutral to water like with Reshiram, or neutral to dragon/weak to water like... well... most 'mons of the fire, ground, and rock types.

In short: without a shake-up to the system, dragon/water will never be a good offensive combo in PVE.

I'd say it has some use. Since Palkia doesn't have a water fast move atm, there are better water attackers out there. That being said... DragonTail + Hydro Pump is by no means a bad combination. Palkia's unique Water/Dragon typing means that it won't be weak grass or electric. If a raid boss that's weak to water but has coverage moves like Solarbeam (Groudon for instance), Palkia with HydroPump would be a great option in that case!

For the time being though, it's not really necessary unless you have both candies and stardust to burn (100,000 + 100 candies is no joke).

My DT/Hydro Palkias were great for the recent Typhlosion raids. I have a ton of dragons powered up already so the double dragon moveset has no interest to me at all. I wouldn't waste the resources on a second charge move however so TM them if you need more dragons.