Benefits are marginal as for now. APO might be a game changer in the future. It works on 14700k and 14900k. But it’s too early to tell. APO aside, you gain only a little bit of performance. It’s not worth the price tag, in my opinion.
Anyway, if it’s for gaming, the 14700k is the better choice. If you undervolt/oc it a bit, you get a similar performance for a cheaper price.
You also don’t need a Z790 unless your plan is an aggressive OC. A Z690 is most likely enough.
And if you buy a new mainboard, consider switching to DDR5. 4 sticks will be very difficult to get stable in OC situations.
You specifically asked for water cooling and I suggest you go for a quality one because cheap ones like to break.
I use a DH15 air cooler just fine, it doesn’t have to be a water cooler for your setup, just saying.