I know that the ocean temple is quite rare, so I think that elder guardians should have stronger attacks and have a lot more hearts to make them a lot more challenging. That way the ocean temple would be worth finding. Let me know what you think about this idea.
I fail to see how you think the Elder Guardian is not as challenging as you would think.
They use different types of powerful attacks, already has a high amount of health (80 hearts), and rare, valuable drops. Sponges, one of the Elder Guardian's drops, are rare and valuable in survival Minecraft, there is no other way to obtain sponges other than defeating an Elder Guardian or finding them in the water temple's "treasure room". Elder Guardians, unlike normal Guardians, can also place potion effects (as well as a creepy picture of themselves...) such as Mining fatigue. Their laser does 4 hearts of damage (8 hearts), as well as a defensive thorn like attack.