I am installing a new water heater out of sight of the main panel. I plan on using
A 30A on and off switch.
Is this up to code or do I need a disconnect ?

You don't need a disconnect.

You DO need some form of disconnect. In your case the easiest (and cheapest) is a common 60A pull-out type non-fused disconnect.

Doesn't the switch count as a disconnect?

As long as the switch is a two pole and is within sight of the unit it is.

He said he was planing on using a 30 Amp switch, thinking he was putting near Water Heater. Was hoping he meant 2 pole as 2 pole switch is only way to secure ALL(both) live wires. I shouldn't assume, curious now if he has 1 or 2 pole switch.

I apologize if I missunderstood. I thought he was asking if he needed a disconnect at all.