A standard 4500 watt water heater needs a 30 amp two pole breaker, 10/2 with ground, and draws 18.75 amps.
There must be 2 spaces stacked vertically in the panel. They cannot be across from each other. A single pole breaker can be moved to the other side to make room for the 2 pole. A 2 pole mini works as well, but it must be positioned correctly.
You have to have two circuits free together. You can use a tandem breaker on one of your single pole circuits to free up some space.
If you already are running three or four other 240v circuits on your house plus going to add another with the water heater, the 100amp panel is pushing your load.
When you figure out if the service will hold the extra 240v circuit, you will need to pull a 10/3 NMB with ground to where the new water heater is going. They are usually orange. Just tell the supply house exactly 10/3 Romex with ground. You will need to sleeve it coming out of the wall too. I usually use greenfield 1/2" with a 1/2" greenfield 90 going into the heater. http://www.foxelectricsupply.com/con...oductNo=ARL851
If you need step by step instructions you can pm me anytime.
Tandems can only be used in the panel if the panel is labeled for use with them. Not all panels can use tandems.
Why did you say you needed 10/3 for a straight 240 unit? There is no need for a neutral. 10/2 is all that is needed.