I need a seat for my ocean kayak frenzy. I seem to need more support because my lower back starts to go numb after a little bit. Would a taller seat help with this?

I doubt it
A taller seat won’t help and might actually make things worse. Those boats don’t come with seats at all, as I recall, they are just rather basic molded recreational sit on tops with a place to plop your keister and a series of steps for your feet. I imagine you would have difficulty mounting an aftermarket seat in one of them – nothing on the hull to reinforce any mounting hardware for something like a back strap.

Back problems are usually due to poor posture and not-so-great paddling form. You should not be leaning back, for one thing. Is there someone you paddle with who can help you with technique? Working on your hip and hamstring flexibility would help too.

But bottom line is, those molded rec boats are not designed for extended paddling – you may have to consider upgrading to a better kayak if you plan to spend a lot of time in it.

back exercises
as others said, poor posture is probably to blame. Keeping your back straight and upright is key. google core exercises for kayaking.

low back side and abs.

and stretching too.