While water is important, I'd say some quality soil and partial shade will make the biggest difference. Organically rich, moisture retentive soil will give you more bang-for-the-buck with water, especially if your soil or your pot is mulched over after a really good dousing.

With some afternoon or partial shade and a quality growing medium, you shouldn't have to water more than once a week, though watering a bit more often wouldn't hurt at all.

Fully agreed, no amount if water in so cal will help a Thai giant that's in direct sun, here I only let mine get morning sun, dig out a good big hole 3' across maybe, and a foot or 2 deep, replace it with half rich organic humus, maybe even some soilmoist or similar product, and half native soil, mulch it good and stand back!
Lots of fertilizer too, start weak and work up, I'd add fert every week, assuming the plant is doing well, if its hurtin, fertilizer will only kill it fast.
Some must at night might help too, and or early morning.
Oh and it needs some room to spread its wings! In your extended season, if it does well , u could easily see a 10' foot tall 8-12' wide beast!
There was one here in ky that got 12' tall last year...

How could I have forgotten!
A more suitable choice for that climate may well be some of the new hybrids released by briansbotanicals!
Noble gigante
Dragon heart
And... Future releases!
I don't know how these handle the lower humidity, I do know these 2 can handle much more sun than the Thai giant, and can still achieve massive size! As one parent is Thai giant, under differnt light the leaves may be green purple black in varies shades and hues, some bright, others dusty, usually combinations of all those, and the future releases he's got under trial look great!
There's also jacks giant, it's a super sized regular colocasia esculenta, it still wants some shade but may do very well for you
I'd order directly from his site. I'm sure he can help answer how to best grow them there too
Best of luck, whatever you chose u can't go wrong, amazing plants!