agree that eliminating the converter should be more efficient.
The exception to this is where you are using an element that is not well matched.
Lets take a really BAD example...
You have a 5kW, 11.5 ohm resistive element (ie, 5kw/240V element), and 2kW of PV panels connected to give 60V output.
In full bright sun, you would think "I should be getting 2kW into my tank"... but at 60V, you're only going to convince 5.3A to flow for a total heating benefit of just 312 watts.
Using a dc-dc converter, or dc-ac inverter and boosting that to lets say 120V, even if your inverter is only 90% efficient, you're going to get 1125 watts into your tank.
1125 watts is almost 4 times more USEFUL POWER, even if you are "throwing away 10%".
You need to look at the big picture before deciding what is the "best" way to do most jobs.